If I'm looking at the main screen correctly, a question arises: Why isn't Beast Girl the canon size?
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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.