I know I'm super late to contribute here, but the raws make it much clearer why Ru goes middle-schooler in the daytime (also, that's what he was commenting on, her getting small in the day, not her getting especially small at noon: "hiru" is both noon and day depending on context). The fans translated it as "mind your own business" for some reason, and the official publishers somehow came up with "treat me special", but the most accurate translation of what she said there would be "show/learn some restraint". In short, he inadvertently convinced her that he couldn't control himself when she looked like a high school or college student, so to get him to hold back in the daytime she takes on a form that's mature enough to work but immature enough to discourage him.