Bottom line is, after "fixing" the left breast, the right one seemed too small, so I "fixed" it too. Now they're both too big.

Hmm, they are a tricky bunch aren't they

Any idea how I can fix her breasts without making them huge? I have no idea how to compensate for the backpack :S
Yeah I thought the backpack would be tricky.
One thing you could try is squashing one of the breasts over a little

If you're still finding size an issue you could try using the liquify tool in PS to save the work of painting over.
Btw, how do I implement the whole "sitting on rib cage" thing? I cant tell if how it should look from this angle.
I think since you painted over it's looking pretty good already, the basic idea was to make sure that the breasts are reacting to what's underneath them and not just being big helium balloons.