New Guppy Mama can give you a better feel for
eventual size, though I would guess them to be smaller that their regular ancestors.

Mollies may have locally different needs in terms of water, but generally it is mineral rich (maybe 200 to 600 PPM TDS or DH (hardness) of 150 to several hundred PPM.) At least one of the (2-3-4-5) mollie species that seem to have been bred into many of our commercial strains of mollie can be found in the Gulf of Mexico (not too deep or they are dinner). But the same species, Poecilia latipinna (one of the sailfin mollies) also lives near Little Rock, Arkansas, certainly not near any sea waters. They must become much less active in winter time.

If you must use sodium compounds, use a marine mix or the much cheaper feed salt (sun dried sea salt) at agricultural supply stores. Rift Lake Cichlid salts would be even better though they are considerably more expensive.

Epsom salts (mostly a magnesium compound) have even been suggested (to raise the specific gravity?) of the water of constipated fishes. So a dash of that stuff wouldn't hurt. I think that is in some of the DIY Rift Lake cichlid salt mixes. (Google search home made or DIY cichlid salts.)

You very possibly know all of the above and are keeping that balloon belly in hard, warm water. You probably also know that mollies seem to be especially vegetarian - hence their toleration of fry. However they are omnivorous enough that they will take most flake foods and certainly chase down the popular live foods used with small aquarium fish.

Fish and other creatures that require a significant percent of their diet as vegetable material will have intestinal tracts that would extend several times their total body length.

(I love a meat lovers pizza - the carnivore's special, but still consideration of the length of the human intestinal tract suggest that we should be consuming proportionately more veggies, fruits and grains than we do.)

The egg shaped fishes, gold fish, balloon belly/ pot belly mollies and other tropical fish (and some natives like the American Flag fish) have arched backs and projecting bellies. They are proportionately shorter that their parent strains. Most of these have been developed by Asian breeders, who seem to favor that shape. I will admit that I like the comet goldfish (longer and sleeker that the original goldfish) better than the "scrunched up" ones.

That's part of the personal preference thing.

On one e-mailing list I'm on even had a correspondent get really upset with a new kind of balloon belly fish and worried about it getting loose in the wild. However these fish swim with a rather distinctive style (which sounds better than with a lack of grace). Compared to their wild counterparts (even black mollies gone feral) they swim more slowly and awkwardly.

Unusual and seemingly wounded swimming patterns really attract the attention of predators who can also concentrate on the unique swimming pattern and not loose the fish in the school. So I don't think that these creatures will be a big exotic problem.

On a NANFA stream survey, at the request of a local nature center, we did bring up a kind of squat, ugly, dim green fish. Someone in the group had been pretty good at identifying the other 26 or so species we found. (I just shut up and listened.) When one of the de-facto team leaders pulled that squat fish out of the seine net and asked what it was, there was an embarrassed silence. He grinned and laconically remarked, "Haven't any of you ever seen a goldfish?"

Not like that! Clearly it still carried a lot of
the egg shape reminiscent of the more specialized and fragile goldfish but it was a sport among fancy goldfish, reverting to a more "natural" color. That specimen's golden and bright silver siblings probable had been eaten by a medley of piscean, avian or raccoon-like predators.

Meanwhile back to your mollie. Just a guess, but it may be suffering from an obstructed digestive tract. (Wonder if that regressive physiology can
interfere with reproduction too.) If you haven't been feeding it veggie flakes and the like, please try. Every now and then someone mentions cooked peas on Guppylog. We use not salt nor butter when cooking green peas. Before dinner, I generally spoon a few into a small bowl or fish room yogurt cup. I did that the other night, gently squeezing the inside halves out of the shell. (Shells can be fed to large snails or in our case, the schnoodle, who was jealous of the attention being received by the fish and would eat almost anything they were receiving just to get attention.

The peas were crushed or broken to size. A whole bunch of livebearers, several killifish (Fundulpanchax, American flags, pupfish, Fundulus notatus) bristlenose "plecos," and even some minnows like southern red belly dace and a Notropis species grabbed them pretty cheerfully.

One could safely guess that the veggie protein in peas will not hurt most fish. The soft bulk may lean out the digestive system, even of "constipated" fishes.

Once took a verbal ragging from college residence mates who cheerfully suggested that constipated fish could be identified when they would swim up to a Val plant, grab hold with their pectoral fins and strain. ;)

Either not passing material or having long fecal "ribbons" trailing after them are more likely indicators.

Feeding blackworms (a very rich food) isn't the problem for most livebearers that it is for Lake Malawi cichlids, who comb the algae beds with their brush-like teeth, or pupfish, which even eat hair algae. In fact, aquarium fish fed almost nothing but blackworms or other worms (great spawning conditioners for a time) tend towards obesity.

The cichlids and pupfish may blow up (Malawi bloat) and even die.

You probably have been feeding some veggie material. Try more. And seek to do more partial
water changes. (Guess what I really need to be doing in a couple of minutes?) Though it is  unlikely in much of the US, but also look to using harder water if you haven't been using pretty mineral laden stuff. Your municipal water department may have copies of the report they are required to send in to the EPA every now and then, Ask for as detailed an analysis of your "drinking water" as they have.