They aren't even unreasonably large for an anime character... besides, we'd need to know her height to really judge, since this perspective is all relative to body size. She could be 4'10"/147cm tall for all we know.
there is just NO WAY to have natural breasts this big and be this light. humans just are'nt built like that
Breats mostly contain one of the least dense material of the human body, fat. That is to say, most of your weight comes from your internal organs and muscle.