In the summer being inside in the day and out at night would be okay, but in the autumn and winter it is too big a temperature difference for buns used to heated houses to go outside at all.
Basically all commercial 'rabbit' accommodation is going to be unsuitable, most is unsuitable for small rabbits, but I havent seen anything I would put a giant in. You could get a hutch or similar, but you'd probably have to have it made to your specifications, which actually isnt as expensive as you'd think sometimes. Rabbit accommodation should be long enough for them to take 3-4 hops, which is around 6-7foot for an average size rabbit, and deep enough for them to lie out straight accross the depth of the hutch, and tall enough so they can periscope with their ears erect comfortably without hitting the ceiling. So as you can see, most 'rabbit' accomodation wont fit the bill for a giant.
I agree that 25x12 is a good amount of space, but I dont have a giant either. I have a large bun though, a healthy weight for him would be around 3.6kg and he was happier in my old house, I think he had about 16x12, cant quite remember the size, he has smaller now. He definately used all that space, it was a brilliant size for him.
As for single buns, I think they should be allowed the same space as a pair, but once you go above a pair you do have to start increasing the space a lot, although its not as simple as 'each bunny needs so and so square feet'. Think of it like goldfish, a single goldfish should have minimum 20 gallons of water, but you can have four in a 50 gallon