If I'm looking at the main screen correctly, a question arises: Why isn't Beast Girl the canon size?
Viewing post in INVULNERABLE V1.02a comments
The obvious answer would be "Because it's canon." even if there might be others.
She's not even in the romanceable rooster so far, as far as i know at least. 'Roadmap' only shows Shrink Rae, Atom Eve, Amber, Debbie, Annissa, and William. So who cares if a character you can't "romance" looks like a twelve years old when she is actually twenty six?
Well she does have a child body in the serie. Even if there is no porn in the serie and that it's a big difference.
I wouldn't recommend the dev to make her look like the original character anyway. A twelve years old looking character in the banner, even if it's not a romanceable character would pose a risk of his game to be banned and persecuted.
I do understand the question though.
Short answer would be "Twelve years old looking body? In a porn game?"
So long story short, 'Beast Girl' looks like an adult to avoid taking the risk to be labelled a pedo and see the game demonetised, blocked and persecuted.
De-aged people, like Monster Girl are not considered normal romance interrest by most people.