Disimulated is on the right track, but overstating things a bit. For instance, a developer can render the following four images and give the player a huge variety of choices:
Render 1: Long Hair, T-shirt, No Piercings, Normal Breasts
Render 2: Short Hair, T-shirt, Nose Ring, Fake Breasts
Render 3: Long Hair, Cropped Tank, No Tattoo, No Piercings, Normal Breasts
Render 4: Short Hair, Cropped Tank, Arm Tattoo, Navel Ring, Fake Breasts
Once rendered, the dev could spend twenty minutes splitting up layers in Photoshop and give the player 2^6 = 64 different combinations with much less time rendering.
So, the major changes, like short vs long hair will double the number of renders in most scenes, while the minor changes, like a piercing or small scar may increase it by little to none.
This means that the developer has to weigh the value of major changes with the exponential increase in the time to render that scene and all future scenes. So, please be understanding if they limit your choices.