Well I thought it would be a problem but 3 times now my 3-5lb mini rex has bred with my 8-10lb american sable bucks. She's had no problems. The mother has more say over the size of the kits than the father so you are increasing the possibility of problems but it by no means is a guarantee it won't work. It's just not a smart plan and it was actually my mom who moved my mini rex from the cage and put her in the colony.
There is nothing to gain from this cross. If you are going for meat rabbits why breed to something so tiny and dainty? If you are looking to put extra meat on your polish, then why something so much larger? A dutch or mini rex would work just as well.
If you are considering showing either then it's just plain silly. Trying to come back from the failures of either breed would take years of work.
They are the breeds i have, no other reason. The NZ doe is bread (so the buck is just sitting around) and the Polsih Buck is not mature yet (so the doe is just sitting around). So i thought i could get another litter this year by crossing them.