IMHO there is a lot going on with horses that have bellies. An overweight horse will be fat all over. Not just a belly.
Possibly your hay is not good forage. There is a lot of wonderful information about how the gut works. We simplistically think hay goes in and goes out, but more is going on than that.
Do they get exercise? Lack of muscle tone can cause some of the issue, I believe. If they are not getting enough protein, then they can lack muscle tone.
Have their teeth been checked? A large part of the horse digestion begins with grinding the food.
Have you had a fecal exam done to determine if they have a parasite load? That is not always the ideal way to tell, but it gives you a starting place. Using wormers, imo, depends on your location and situation. My horses are on rotated pasture and do not go to shows, so I feel their exposure to parasites is minimal. I have them checked twice a year, fall and spring. Usually they are only wormed every other year! I feel a wormer is more potent if given less frequently. A wormer works not by killing the parasite, but by administering a neuro toxin that paralyzes the parasite, which causes it to release its hold and be expelled. The dosage is designed to be enough to affect the parasite without harming the host. The parasites die after they leave the host. I always clean up manure in my lots.
You can search this forum for lots of information about your topic.
So good of you to ask questions and be concerned about the horses in your care! We are always learning.