I don't know a lot about mares/foals/breeding and whatnot, but if that were a cow, I'd say that Maggie had had a calf at some point in the past, so maybe Maggie has had a foal at some point. My new mare has a largish udder, and she has had foals in the past. Daughter's stallion got to her right after I brought her home (dangit) so I have a feeling I'm about to learn more than I want to!
When I got her home last week, her udder didn't look as large as that. In fact when I bathed her on Saturday it didn't look that big. Yesterday when I was applying cowboy magic (the cream fly repellent) she about bounced all over the place when I went to apply it on her tummy. I looked under expecting some kind of wound from that reaction and the enlarged udder is what I saw.