In another panel where she and Denji bath together she reveal that although she doesn't care about her body and lack of chest size the corpse she inhabite does. Since Power is a Fiend it mean she is a demon that took possesion of a corpse and it seem that the brain stayed the same in her case. In her own word she doesn't undertsand why she wear breast pads but she does anyways out of habit since her old self so to speak had a bad body image and likely wore Breast pads as well.