Mine didn't go back entirely. Before becoming pregnant with my first I was a 34B. After delivering him, when my milk came in, I topped out at 36E. After two years of breastfeeding I settled finally to a 36C and was perfectly content with that. Now 17 weeks with my second and already having to move up a size, we'll see what happens this time!
A really good supportive bra is key
Make sure you are wearing the right size - ESPECIALLY while nursing. Most people aren't and carry the weight of their bust in their shoulders which causes shoulder and neck misery! And when your bests are heavy with milk this can equal a lot pain. HerRoom.com tells you how to measure yourself properly. A good bra that actually fits is a woman's best friend!!