Kankin Idol Recital ~Idol Ryoujoku Nikki~

Yuka-chan is an idol with small breasts, so she faces fierce competition in the world where big boobs rivals. Main character is a number one member of her fan-club. He learns about a secret thing between Yuka-chan and her manager, so protagonist abducts and confines the girl. This confinement life will go on until the girl realizes who her only true love is.

[From vndbreview]