I have lost 13 lbs so far and I did shrink in my chest (thank GOD!!!) but I can't remember how much exactly. 1" for sure, possibly 2" (I have the measurements at home). I measure myself across the boobs, not under bust. Despite the shrinkage, I still wear the same bra size (and no need to get a smaller one). I think I would gladly make a donation to church if I shrank from a D to a C cup (in spite of being an atheist).
I have definately lost in the chest. From 42DD to 34C. Hopefully they'll hold right there. Of all of the times I've lost weight it's never been very significant in this area...so who knows??? The body definately has a mind of it's own!