I'm kind of amazed this hasn't been brought up here before... I only just noticed it because of the thread in the Growth section, but it deserves a spot in the shrinking threads. If I've just overlooked it, I apologize.
Two characters, a normal woman and a giantess, are each shrunk down by a magic mushroom. The former then spends most of the rest of the day in the latter's cleavage. Yeah.
The content begins in
Chapter 30, wherein the two women are seen normally sized and then, after shrinking, later sneak around a fighting tournament incognito. The process can be seen
here via flashback. The jig is up in
Chapter 33. They gradually grow over the next few issues.