Are mares that have previously had a couple of foals prone to being fat later in life? I know this is a very odd question. She is in work 6 days a week, living out and not on any hard feed - 13 year TB x ID by the way
My cb x aa mare had a foal at 3 (!) before I owned her and she was prone to having bit of a big tum even when fit. She had another foal while I owned her which is now rising 3 and the 2nd baby seemed to make her a have a bit more tummy too but it could also have been that I didn't have as much time to ride her.
A mare that is out on grass full time will have a bigger tum than a horse that is in, it's something to do with the way they digest grass that gives them a fat tum.
Having a foal has a tendency to level out the hormones which means the mare will feel more content herself so she will digest her food more efficiently. For this reason there is a possibility she will carry more weight. Its doesn't always work like this though - some Mares will always be a little neurotic regardless of having babies!
My mare (IDxTBxApaloosa, now aged about 12) was 'rescued' from a life of having foals, at least 3 I think that are known about and she had to be sewn together again 'down there', and she has a belly like she's going to drop another one any minute. She was actually scanned by the 'rescuers' as they were convinced she was pregnant but it was negative. No matter how fit she is, what diet she is on, she still has the belly. I did ask the physio (who was visiting another horse) if there was much we could do about it and she said that no, the mare was going to stay like that pretty much, her ribs had spread, muscles changed etc. It doesn't affect the mare's performance and she's not much of a head turner anyway so the belly doesn't let her down that much! (bay in my siggie - I never take bad angle shots of her if I can help it!)