Simplest way to increase boobs is to wear some cow gear, especially the ears.
Desire, submission, exhibition, masochism and libido are affected randomly by some enemies.
You could get a suggestion to try to get one in specific higher (but they cost 200 mana).
Also there's some curios that affect them randomly and others that target one specifically.
(There might be another way to affect them that i don't remember right now)