The body type system is mostly a style choice; I wanted girls to have labelled body shapes such as hourglass, pear & apple. There's also dialogue and clothing unique to particular body shapes.
But in terms of sizing, it does actually already work a bit like you describe. Each character has eight body part sizes: neck, arms, chest, waist, belly, hips, thighs & bum. The gain patterns determine the basic ratio of these, but there is also a random element, such that not every apple-shaped girl will gain in exactly the same way.