i find it depends on the rabbit.
it is true that multiple bunnies will consume more, but I find that multiple bunnies grow more quickly.
singleton bunnies eat less, and grow at a more reasonable rate.
So it's a catch 22, do you want the frame to build more slowly, or do you want lots of feed and faster growth? For my breeders...I want them to grow more slowly so I'll have no more then two in a cage. Take your time growing up, that way you are where I want you to be mentally, when you are big enough to breed.
But for my dog food rabbits. I'll stick you all in one cage (divided by sexes) because once you hit four lbs, you're ready to go. That leaves more room for the others to grow out and then if there is one that surprises me, then I hold it back. Often they hit four lbs close to the same time and that leaves room for any slow growers who with more room and less competition are done in the following week while consuming less feed.