Here's an incomplete list of things that feature breast reduction in some form or another:

Games:

As a mechanic/feature:
- Pai Touch! (available in almost every route)
- Omega Labyrinth (flat chest status effect from trap and equipment, cup size reduction by trap)
- Omega Labyrinth Z (flat chest status effect from trap and equipment, cup size reduction by trap and certain monsters)
- Genkai Tokki Seven Pirates (breast size/shape sculpting via masssage minigame with cup size indicator)
- Genkai Tokki Castle Panzers (breast size/shape sculpting via massage minigame for each class that each character can switch between, no cup size indicator)
- Maebari Teikoko no Gyakushuu (available in one route)
- Sex Life (original Sano Toshihide eroge, can customize heroines to have smaller breasts)
- A number of DMM browser games
- Mahou Shoujo Mayu ~Suitsukusareru Maryoku to Minamoto~
- https://game.nicovideo.jp/atsumaru/games/gm4468 (bad end, second option when talking to plant girl)

As DLC:
- Gal Gun Double Peace (costume option applied to all female characters)
- Senran Kagura 2 (DLC side story mission and customization option)
- Bullet Girls 2 (DLC option for interrogation mode)

Anime/Manga/Light Novels:

- To Love Ru Darkness (Episode 7, Chapter 10 in Manga)
- Kamen no Maid Guy (Episode 6)
- Manyuu Hikenchou
- Toshiue no HIto (Chapter "Stolen Breasts")
- High School DxD (Vol 11, Life. 4)
- Kanokon (Episode 9, Chapter 30-34 in Manga)
- Mirai Nikki (Episode 25 Omake)