Skin is the topic that so far I just haven't touched, I'll try to figure it out.The skin render looks a bit weird.
This render is a WIP I'd say, that's why I haven't posted it on deviantart and pixiv. Honestly, I don't think big breasts would be a hindrance in being a fitness model but point taken on the other two aspects.I don't see her as a fitness model, I'd say that her body doesn't fit because her arms are too thin compared to the rest of her body and her body is not defined enough to be a fitness model, also I'd say her boobs are too big but for that, you can say that she has implants.
Personal point for me her chin is too big for her face.
The render itself is a bit grainy but I think that is just you being new to rendering.
Yeah I see it. But I don't know how to fix that.
As for the render, I don't know how it became this grainy, maybe changing the aspect ration will fix that.