Swollen abdomens can be from any of several causes.
Livebearers are the most likely to be pregnant.
Fish that lay eggs can get big when the eggs are developing.
Either condition is usually symmetrical, that is, it looks the same on both sides if you are looking straight down at the fish. It can be a bit lumpy.
Eating too much at any one meal can make a bulge in the fish that goes away as he digests the food. But that is too much food. Spread out the food better so all the fish get some, and one fish cannot eat that much.
Eating the wrong foods can make the fish constipated.
This will also look like a symmetrical lump, but usually a bit farther back than the 'ate a big meal' lump.
The solution to help once they are constipated is to move them to a quarantine tank and add Epsom salt.
The prevention is to feed a rotation of foods that include roughage. Daphnia are good, and vegetables like Peas. Do not over feed. Small amounts more frequently are better than large amounts infrequently. Feeding less often is best. If you feed twice a day, then skip at least one day per week. And only feed tiny amounts.
If you are raising fry keep them in a separate tank so you can customize the feeding for them. Lots of food, pretty often, and LOTS of water changes.
Fish can get tumors. These are more likely to be asymmetrical, and can be anywhere on the fish. It would be REALLY weird that several fish all got some sort of growth at the same time.
Internal parasites can also make the fish swollen. Often these fish will be lethargic, and hang out near the bottom of the tank, but not always. The treatment is to worm the fish.