What's wrong with having them just be bigger?

It's not like it's suddenly a whole new species. It's a specimen of that particular species that has grown bigger, stronger and more vile over the years and therefor has to be put down.

Girtab is still just a mite, even though he's supposedly the strongest of the mites and therefor was named.
Sabotender Bailarina has to be put down because it has grown a flower and turns other cactuals into a frenzy around it.
Myradrosh has grown so big it started feeding on anything with a red color (i.e. anything that bleeds) and not just red flowers.

A lot of the monsters already are not just bigger but have something unique to them, as small (read: realistic) as it may be.

It makes absolutely zero sense that you have a group of monster species that we're going to call A, and suddenly one monster of that species has 20 horns sticking out of him, bright blue colors and flames, just to give an over the top example because it has to somehow be more unique.

I think the developers did a great job keeping it true to the game and it's lore. There's a crab with a Giant's helmet on his back that he's using as his shell, bigger and more notorious specimens of existing monsters and even some bandits and beastmen that were so out of control that they had to be put on the list and there's even one albino squirrel. Nothing out of the realm of possibilities. They might be minor alterations from regular monsters, but i'd much prefer what they did over having them be rediculous.