Onee-san Ha no Ore ga Naze Chicchai Kimi wo Suki ni Natta no ka?

Story

It was spring break after middle school when Sky Toochika rescued a tiny girl in a packed train, as she barely even stood a chance against all those people. When he starts high school, the grade-schooler he thought she was turns up right here in his very classroom!? Her name is Chito Naruse: rather small, but drop-dead gorgeous—oh, and also a total energy addict. Then the little Chito-chan hits Toochika-kun straight up with a confession!? "I'm a devoted fan of tall, older onee-sans—so a cute, pint-sized girl like Chito-chan is not exactly my type.

Synopsis via AniList (English).