Maybe try fasting him a day and see if he looks smaller. Could just be stuffed with food or it could be parasites.
If you have females around they could have recently dropped fry and he could be having his fill on them. I have a betta with platy in my son's tank and I know immediately when we've had shrimplets or fry in the tank as he's a pig and stuffs himself on them then refuses to come to his feeding spot lol and rather keeps hunting with his giant gut looking for more.
That is a ballon molly. is supposed to be like that. I don't remember if those are called sailfin or lyretail ballon molly.
I’m pretty sure ballon molly already look like balloons at the time of purchase and The OP says that this size is abnormal for this fish.