She could foal tomorrow or a month. Some mares are A cups while others are DD cups when they are done bagging up.
As far as a vet saying that she is not likely a maiden, that is only a guess. My mom took a mare in to an equine repro vet that said the mare was a maiden because of how the uterus was shaped. Well, progeny records say differently, she had a colt three years before this breeding exam. LOL