A few years ago I read a doujin of this demon girl that appears in front of the mc as a loli but he says he isn't attracted to her since she has no breasts. She then transforms older with breasts and he's all up for it but i think it takes energy to sustain that form. I also think she is a succubus and she transforms throughout the doujin. I do remember that in the end he tells her he loves her no matter her breast size.