Hey, understandable. I'm an erotic writer and my preference is big booba (despite being flat-chested myself). When I write more two-person romance stuff, I tend to stick to a curvy, busty leading lady. But when I write a larger cast, I try to give more variety both because it helps me differentiate the characters better and because I can write more variety of scenes involving things that might be unique to the different body types. But that's just what I do, and what I prefer, and I would never begrudge anyone for writing or creating what they like or are comfortable with.
And hey, props to you for properly recognizing the concept that is "medium-sized breasts."