Yeah, that's one of the things I don't get: how some woman so weak-kneed and having all kinds of problems (including, apparently, anemia) and with weakness in every aspect of her life can have a voluptuous body like that--hard to "suspend disbelief for the sake of entertainment". Maintaining a body like that takes exercise and she certainly doesn't do that because she's so weak, so one of two things is going to happen: she would either wind up being grossly overweight; or if she's not getting enough nutrition, she would wind up abnormally skinny.
Really, the ecchi-ness and the fact that the artist can draw women really well is about the only good thing about this manga. Otherwise the plot is kinda stupid.