OK, so i'm PRETTY sure the GTS's name is Traci/Tracy. And I'm pretty sure the guy's name is Tim. (not entirely sure though) But anyway, in the story, traci is good friends with tim's little sister and she knows witchcraft or something. the two girls are hanging out and they decide to look inside tim's mind to see what he's thinking. he just happens to be masturbating and they find out he's thinking about being in traci's cleavage, (i remember she specifically said "the size of a mouse" and traci said "in my bra? you mean while i'm still in it?")
traci successfully seduces tim and i think his sister casts a shrink spell. he gets smaller and smaller and eventually traci breastfeeds him. he may or may not continue to shrink and simply seep through her skin at the end.
i know the information is kind of vague, but this story definitely sticks out in my mind, hopefully it sticks in someone else s as well. thanks!