Aneki... My Sweet Elder Sister: The Animation
Takumi just cannot stop fantasizing about Saki-senpai, her voluptuous body just stands out anywhere. One day she talks to him on the train and they become friends. Whenever he can, Takumi stares at her form, always getting a hard-on. The next day he finds out they have the same fetish… and so the story begins.
(I didn't make this description)