Not really true. It is said that the male shouldn't be overly larger, like twice or three times the size. Bucks that are larger than the does are used for certain goals, like in meat breeds, I would say 1/4-1/3 larger is quite common. I actually haven't ever heard of actual problems due to a bigger buck, maybe because nobody tries. One theory is that kit size depends solely on the doe since it's her resources controlling that, and the genetic size comes into play when the kits grow up.
If you would always use a smaller buck the rabbits would get smaller and smaller over the generations.
Anyway, I can say nothing definite about dwarf breeds, and how that is handled there, my guess would be that since true dwarfs can be bred it wouldn't make any difference if the buck is false or true dwarf since you get false dwarf kits anyway. You just avoid peanuts with a mixed pairing. If the size of the false dwarfs in the respective lines is in the same range there wouldn't be any difference, right?
But as I said, no own experience with dwarfs.