Mini Rex and Lionheads are what are known as dwarf breeds, meaning that they employ the dwarfing gene to get the tiny, compact animal described in the breed standard. Unfortunately, the dwarfing gene is a lethal gene; any rabbit that gets a copy of it from both parents (referred to as a peanut) will be deformed and die, usually within a couple of days of birth. Virtually all of the purebreds of the dwarf breeds that meet the size requirements of their respective breed standards have one copy of the dwarfing gene, and one copy of the normal growth gene. When breeding dwarf rabbits, one of the possibilities is a rabbit that didn't get a dwarfing gene from either parent, but instead got the normal growth gene from both of them. We call these animals "false dwarfs," and they generally grow to be half a pound to a pound over the showable maximum for their breed. You can usually spot them easily; they have longer bodies, longer, narrower heads, and longer ears than their "true dwarf" siblings. Looking at pictures you posted of Heath and Clover on another thread, their proportions give me the impression that Heath is a false dwarf, and Clover is a true dwarf.
Obviously, there are other genes besides the dwarfing gene that influence how big a rabbit grows; even false dwarfs are waaayy smaller than the commercial breeds. Breeders have to watch the size, even of the true dwarfs, because they can creep upward or downward over generations. Mini Rex are one of the worst for this; Rex breeders refer to an animal they call a "Midi Rex" that is much too small to be a standard Rex, yet much too big to be a Mini.