I can understand your concern and certainly it's possible, but it's not probable. Is this her second foal? I ask because some folks, I don't know if I've personally seen this, swear the first foal is always tinier and subsequent foals are larger.
I wouldn't worry too much. She could just be carrying the foal side-to-side right now - the baby laying with head on the right and butt on the left. That makes them look huge! But once the foals move into foaling position (head toward mare's rump, butt towards mares shoulder) they slim up from side to side.