Mares carry their babies differently, just as people do! It will depend on the number of pregnancies and how "stretched out" they are, the amount of fluid they're carrying, etc. Just like some people look like they are carrying just a basketball -- and others look like there is a whale inside! (Personally, I had a lot more WHALE pregnancies -- and all of mine but one were born 2+ months early and premature -- but my daughters each looked like they had little basketballs, and one little basketball grandson was almost 10 pounds!!)
Each to their own -- as long as those precious little ones are happily and safely brought to the ground!!