Making giant rabbits with a commercial type is almost self-exclusionary. Commercial type involves a high percentage of meat packed compactly onto a medium-bone frame. The meat breeds have been heavily selected for both feed-conversion efficiency and the most meat on the least frame. It makes sense that commercial meat breeders would have bred the very biggest rabbit they could while still remaining efficient.
And it seems that anything over about 12 pounds requires heavier bone to support the increased weight. If you breed a really big rabbit it'll either have heavy bone, which costs more in terms of feed and development time, or you'll end up with bone development problems, which is obviously a non-starter.
Quite a few people around here cross Flemish with NZ/cals/Satins in hopes of "bigger meat bunnies." They generally get what I did when I did the experiment years ago... the Flemish cross bunnies look bigger at butchering (and boy, they eat bigger too!) but when dressed out, I actually ended up with more meat from my purebred Satins than from the big-looking Flemish crosses. The Flemish gave size mostly in terms of bone, pelt and ears!
You might want to develop a giant rex simply in hopes of getting a bigger pelt. But you'll still be looking at that issue of heavier bodies needing heavier bone, so you'd have to be willing to sacrifice dress-out percentage and feed efficiency, as well as growth and generation time. Also, with a heavier rabbit you'll be looking at potentially even more likelihood of sore hocks.
