Let's take a look at each concern in turn...
Are there any disadvantages in carrying a 5kg tent other than needing stronger legs?
Absolutely not! In fact, there is a compelling advantage to carrying a larger tent -- the interior volume goes up faster than the weight (so long as construction is similar). A larger tent is less weighty for the same volume than, say, a couple 1-person tents, and you have the added convenience of extra space for a pair of people and even for each one.
My friend's Tatonka Alaska 3 is within a centimeter or two of the length of my '89 Honda Civic automobile -- even larger if you count the lines. We filled the bulk of a car-tenting pitch at a "campings" outside Ghent. You can see from the interior shots, the screened sleeping compartment ends at the side doorway. All the rest is unfloored vestibule (we used a groundsheet that made a floor for it), with a second door in the end. The interior shots also reveal another Truth about tent capacity -- at full rated occupancy, there isn't much room. A third person would have had to reverse their bag and smell our feet in stereo while we all punched and kicked each other with every change of position. Ew. For two, the sleeping compartment was roomy and the vestibule ginormous, big enough to easily hold a bike. Two bikes with care, I think. We left the bikes outside and used the vestibule to hold all our gear and to prepare food (cooked outside
on a well-mannered butane cartridge stove while sitting partly-sheltered in the doorway!) and pack under shelter. I could easily kneel in the tent, which made it easy to change clothing. I soon came to consider it a small "campings huis
", rather than a tent.
My friend carried this tent on his solo tour from the Rotterdam area to Santiago de Campostela and back last Spring and it served him well.
The carry-weight of a large tent can be shared. One of the great advantages of touring with someone else is you can split your load among your shared gear, each carrying less than you would alone. On our BE and NL tours, my friend carried the tent and stove, while I carried the tools and food. We each carried our own personal items, like clothing and sleeping bags/pads. Carried weight was pretty equal for each of us. When you go solo, you've got to carry 100% of everything, which is one reason why I have opted to go for a smaller, lighter tent. Because I rarely have a partner, I wish to make things as light as I can 'cos I have to carry it all. Setup and takedown of a big tent is so much easier with a second person to tension it out and do their share, especially if conditions are miserable. On the post above where I mentioned "wearing" the tent as I set it up and take it down...that is one of the adaptations I've evolved to do it alone. Working with a second person, a large tent erects and stows in a fraction of the time it would take a single person to do the job.
I would imagine packing it in a dry bag and placing directly on top of the rear rack without any problem
<nods> Pretty much, as you can see in the accompanying photos. I probably would have used Arno webbing straps to secure it rather than the x-pattern of bungees my friend used, but it worked well. He has a Tubus Cargo rack on his lovely Avaghon Series 26 bike, and it rode fine.
You do raise an interesting point few people seem to consider, and that's putting the tent in a dry sack. I have come to think it is better to put the inner sleeping compartment
in a dry sack, leaving the outer fly to breathe in a permeable sack to prevent mold and mildew if the tent has to be packed wet. If it gets rained-on in transit, it is none the worse for wear. In recent years, I've devoted a lot of thought to what should/needs to be kept dry and what can/needs to be allowed to breathe, and have concluded so long as the inner tent remains dry, the fly is probably best allowed to dry on its own rather than steam in a waterproof sack. <-- As with most things, there is no "one right way", and everyone packs differently. Some prefer to place the tent inside
a pannier. I prefer my sleep system (bag, pad, pillow and silk liner) in one dry sack atop the rear rack (un/packing it in the dry tent). My tent also rides atop the rear rack, but in its own sack. My friend prefers to carry his bag and pad and liner in a single Ortlieb front roll-top pannier dedicated to that purpose. Either/any way works as well; it is up to preference, convenience, and personal quirks. When trying something new, I carry a little hand-drawn map showing the new address of moved items so I can find them until I learn where they live.
A large tent can weigh a lot -- even more if stowed soaking wet -- so like any load, it pays to use care in placing and securing it. So long as you have a sturdy rack and handling isn't affected adversely and the overall load can remain balanced, there isn't a problem. There is where it can be helpful to use front panniers as well to better balance the load fore-aft, but they add their own weight as well as that of a front rack to carry them. They are also an added expense.
I feel compelled to ask what is probably a very silly question i.e. does the volume or weight of the packed tent mean compromises in other kit you can carry?
Not a silly question, but a very good one! The answer is "yes it can". As with anything extra you carry in either volume or mass, the space and weight require at least a redistribution of the rest of your load and some care in securing it...securely. In the case of using a large tent and traveling as a pair, the problems are minimized -- except for personal items, just split up the shared overall load. I saw a number of paired cyclists in Belgium who did it this way: One person took the big tent atop his rear rack. The other took two sleeping bags atop hers. They each carried their own pads. Pretty much the same individual load as two of me would take, each carrying a 1-person tent. The difference is they could share the space and had greater overall volume to live in and store their stuff.
are there risks of strain on luggage, racks, or even the bicycle?
I agree completely with Julian here. Bicycles themselves can carry enormous loads (think: cargo bikes in India); the downside is there is more weight to haul (not very noticeable on the flat once rolling and at speed; you'll be wishing for a winch or windlass on 20%+ grades) and handling can suffer. As for the bike, so long as you have quality kit and overall weight (including youself; you're also cargo to the bike) is secure and not excessive (common sense), the bike will likely do fine. I agree with Andy Blance that rough roads make a difference to parts life, longevity, and handling when carrying enormous loads, but as a practical matter, you'll likely do fine on reasonably smooth roads. In the case of Little Tent versus Big Tent, we're talking a difference of only a couple-three kg, and that's generally not enough to make much difference. Julian's right -- Christmas-season goodies can account for that (me!).
Of course you can always go solo and take a large tent as well. No harm done. So long as you have room for your other basics -- sleeping bag and pad, clothing and tools, food and maybe a stove and pots to cook it, you're set. On a short trip, you'll have plenty of room for those. What really kills space is the extra stores of food and water when traveling self-supported in the back-of-beyond. If you'll be reasonably near shops and resupply, you'll be fine with carrying the bulk and weight of a bigger tent. Too often, we get hung-up on having the "perfect tool" for the job right off the bat, when simply using what we already have can make it possible to camp and learn what would work better. It can save making an expensive mistake. I started out with a bright yellow plastic tube tent that cost me USD$8. Sure, it had deficits and I quickly learned what they were, but the thing is...I was outside, bike-camping
(!) and having the time of my life. The first "real" tour I took, I went with a friend to Washington State's rainy San Juan Islands. Before leaving, we seam-sealed the bottom portion of his "waterproof" single-wall 2-person tent and he promised to do the rest, but forgot. He remembered forgetting during a night-long thunderstorm when the top leaked and the bottom held water fine. We had to unzip the door to let water out as the world turned the same Safety Orange color as the tent with every lightning flash. And, y'know what? Though temporarily miserable it was lot of fun. We got dry and lived through it; the memories have remained as bright as the tent color and I can still laugh at our naivete. Mt. St. Helens was erupting, we got lost while hiking and came back to find crows and squirrels had eaten all our food (foolishly left in the open atop the picnic table), and the weather was horrible. As Gilbert K. Chesterton said, "An adventure is only an inconvenience rightly considered. An inconvenience is only an adventure wrongly considered". So long as it isn't life-threatening and no one gets hurt, it's fine. Makes for good stories, too.
Here's another way to look at this whole issue -- deciding what to take is a matter of gaining experience and preference through experimentation, and that's what overnighters, weekend trips, and the backyard are for.
Each of those shortens the learning curve for what works for you, and can substitute for any number of longer trips where you might have discovered the need for improvement on Night 1 of 30 scheduled for the trip. Yes, I stick to the proven setups for Big Trips and Expeditions, but I got to know what works in those situations by playing around. If you take an overnighter and not everything works...it's not the end of the world. Take that lovely (it is!) Hydra out of retirement and give it a whirl. You'll likely have a lot of fun and discover it may not be the answer for Everything, but could be just the ticket for some trips where you want to treat yourself to a little extra space...say in the rainy Fall or Spring shoulder-seasons when the weather is unsettled and you're more likely to be staying in it, or for trips where the formal campsite is central to a destination, say where you'll be using it as a base to explore the local sites or hike out from. One of the things I so enjoy about cycle-camping is it gives me a chance to once again be the kid who played in homemade "Forts". I firmly believe Play is central to learning, and this stuff should be fun, right?
As a side note...perhaps Mrs. Relayer might be interested in cycle-camping with you if a particular trip could be tailored to address her needs. That might be a different trip than you'd take on your own, but could still be fun. I helped plan a trip for a pair of friends where one did not
cycle-camp, and they had a wonderful time. They car-topped the bikes to within 8 miles of their camp one evening, then rode just that far and stayed overnight in a large tent the carried. My friend took care to choose a campsite close to a good restaurant, and they changed onto nice clothes to eat. Next night they kept the tent stowed and made it all the way to a B&B another 10 miles away and spent the day sightseeing and window-shopping on foot. Return trip was to the first campground, but with an afternoon at a nearby spa, then on to the car for the drive home. Best trip ever, they both declared. The biking was at once central and incidental to the overall experience. As for camping on one's own, check-in calls during the day and a goodnight call to the spouse can do wonders toward making it possible.