in a league of her own
Obligatory caption of "Milk and Cookies."
I bet Santa Claus has her in his nice list and would love to share cookies and milk with her. 
While I don't know her story, I would think this would be gestational macromastia, which would mean there would be lactation at some point. When you encourage milk production by draining the breast dry at least a few times a day, it is known to increase production of connective tissue, thus accelerated expansion in size and fullness, the extent of which is individualized. But she sure is looking happier as her breasts have gotten bigger.