@KazumaLee94: From what I've seen, it seems that Oomori was harassed a lot by boys as she grew up, because of her size; they treated her like a "giant girl freak"--typical elementary school crap. Once she got into middle school, boys would harass her in a different way, likely not just because of her size, but also because of her
sizes. It can be assumed that much of the time while growing up, she'd wished that she was small, petite, unremarkable, etc., so that people (guys) would treat her normally. I would say that many manga stories, it's typical that a character like her is really into cute and little things, and this extends to 'senpai'. He's just so adorable to her that she can't help but fawn all over him. Naturally, since he's small for his age, and she's big for hers, she feels a sort of connection with him, though on the opposite side of a spectrum. I don't believe she treats her senpai like a child, especially with how sexually assertive she behaves towards him.