I know you're speaking specifically of the characters' physiques, but as a general idea, having characters stand apart from each other and match who they're supposed to be can be a tricky thing. For example, one of the hardest things for any writer to do (including obviously devs who write their VNs) is to give each character a unique voice that matches who they're supposed to be. For instance, a 15-year-old with low self-esteem would obviously sound much different (in speech patterns, in topics they discuss, etc.) than a self-assured 35-year-old boss babe who's on exactly the career trajectory she's planned for herself for years.
Understanding how to convey the character as they actually are is a challenge since most people who write tend to use their own voice and not that of their characters. And then, to distinguish characters from one another, the writer has to repeat the process of getting into that individual's voice over and over again... for every single character. There are, ofc, some shortcuts; a random passerby only seen for the two lines they speak can generally use the writer's own voice without readers really noticing. But you can start to understand why this is a struggle for so many writers.
This would extend somewhat to modeling of characters as well. Certain body positions, facial expressions, etc., tend to be fairly unique to individuals. Even something like a smirk will differ in things like the angle of lips, how open the mouth is, whether it typically is done with the left or right side of the mouth, etc. You get the idea. All of these body adjustments, especially when taken together across an individual's entire body, become fairly complicated to make each character feel more unique.