itch.io

There is intended variety of character ingame, some get a lot, some get a little. There are also different ‘rates of growth’ where some start fast, but slow down, and others start slow, then abruptly speed up. Characters that do have a final rank, have that held back until the full release.

As Bareon said, variety is part of the reason.
There are other factors though! Fairy is one of the very first Monstergirls I drew, and my standards have changed a bit since. Several Monstergirls also got in through the Patreon polls which guarantees a certain minimum size at higher ranks, but Fairy was added before that. This is also the case for Mud elemental for example.

I will be holding alt costume polls in the future where winning Monstergirls will get a different costume as the name implies. I might very well do an alternate boob size for say Fairy if she ends up winning one of these, but her default sprite will in all likeliness stay as it is!