Some women do actually gain weight in their chest, it just sounds like you're not one of them (I'm not either - I joke that my bra size is like batteries... AAA). I wouldn't think that the weight coming off your chest was necessarily related to running, either. That may have just been what was next in the fat loss "list" for your body, since there isn't really an effective way to "spot lose" - or lose weight from one particular place. You can spot train - or build muscle in a particular place - but the fat comes off at its own pace in its own order.

I know this doesn't completely answer your question, but your genetics do have a lot to do with where you gain or lose weight first. Hopefully you can find solace in the fact that you are losing weight still, even if it means you have to shift down a cup size.