There is an option to do that within the game already. Navigate to the Kinks menu on the main pause screen. Under that is a subheading for Scale. You can adjust any entity's overall body size, as well as a few individual parts. Breasts is the 4th option.
Edit: Realized you asked about making them smaller and they start at the smallest size. There's another toggle for chubbiness under the character menu, which may have a minor effect on making them smaller.
