Your scale will be correct as to weight. However if one was to bone out the rabbits, then weigh the meat then the bone, one would soon realize the bone will weigh more for the Giant X than the pure Cali. or NZ rabbits and less meat. Also, they will eat more feed.When crossing rabbit breeds ALWAYS breed smaller breed buck to larger breed doe. The reverse of this can cause kindling issues, even death for a doe trying to kindle kits that are just too large for her body & birth canal.
On a related note, one of my best does is a NZ/Giant Chin cross. I've crossed her with Silver Fox, Californian, and NZ bucks and have really liked the kits she produces from all those pairings. I'm sure there are some inefficiencies in there if you really had the numbers (I don't), but to my eyeball and kitchen scale, her kits are bigger at slaughter than the Cali/NZ or purebred kits of the same age being harvested at the same time from the grow out pen. Not a lot, but noticeable. But I agree with other posters...I'd never breed her back to a giant breed like FG for meat purposes.