Naturally they produce eggs which grow to a certain size and are stored until fertilization. However the size of the yet to be fertilized eggs may be different from species to species, And the process from egg to fry is rather different for different livebearers.
Female guppies and mollies, platys, swordtails, variatus and Poecilia "whathaveyoui" actually lose a number of bones reaching from their ribs to the anal fin when they develop female features. (There is a drawing of this in the latest ALA Livebearers #179) That allows the females to have room in an expanding gravid area for fry.
Males need to keep that pelvic girdle of bones in order to move that old gonopodium around. That is why some livebearer experts question whether a female could change to a functional male because (and hormonally some old ones take on male characteristics - larger fins, some more color, a thickening of the first anal fin rays) they still would not have the bone structure necessary to effectively use a modified anal fin.
Some Poeciliids (the livebearers we are most familiar with in the hobby, but not the only family of livebearers) develop eggs to a certain size, the eggs are fertilized, the cells divide into little fishes and those are born with a body mass a little less than the egg had when they were fertilized. (That is just like what happens to the fry of egglayers'.) The female doesn't seem to have contributed any or much additional nourishment to the fry. Guppies fall into this category. (There must still have been an exchange of O2 & CO2 though.)
Swordtail newborn fry seem to be a little bigger (by weight) than they were as eggs. Yet mom has no fancy placental nourishing system. Something must be exchanged through the walls of the chamber in which the fry are carried.
The reason some guppy fry are indeed bigger than others is probably that the female of the larger fry (and larger eggs) was feeding especially well (probably included in her diet were blackworms &/or whiteworms and maybe daphnia and/or bloodworms and trace elements and vitamins from some veggie material).
She may also have been a big girl to start with. It follows that guppies in an larger line might produce bigger eggs.
In other cases there is clearly some help from the mother because the fry masses much more than the fertilized egg did. There are all sorts of levels of nutrient supply, depending upon the species.
The champion nutrient provider among Poeciliids, according to Ch. 1 of John Dawes' LIVEBEARING FISHES: A GUIDE TO THEIR AQUARIUM CARE, BIOLOGY AND CLASSIFICATION, is the little bitty Heterandria formosa. The "least killifish" - a pretty mis-leading name for a little jewel also called the mosquitofish (one of several livebearers given that title) or formosa or just "het" is (at last count) the smallest vertebrate on the North American continent.
Even though female H. formosa are much bigger than males, they don't have much room for fry. Full grown males are only a little over 1-1.5 cm in length. A female Het placed 1st in a show class of livebearers at very close to 2.5 cm/ 1 inch long. She was a giant! (If I was impressed with the fish, I was even more impressed with a show judge who seemed to really know livebearers.)
So what the female formosa's system does is allows an egg or two at a time to be fertilized by the sperm stored in her body. Internally she must somewhat resemble an assembly line. She will only drop a fry or two or three at a birthing.
The newborn will be over 3,900% the mass of the egg when first fertilized! In the meantime, Moma can carry up to 9 batches of fry in various stages of development. (Superfoetation at it's finest!)
Another family of livebearers are the Goodieds. Their fry can be huge and are also several 1000 times bigger than when they were first fertilized. (Theirs is a one time fertilization per batch of fry though.) They are actually nourished with a placental type arrangement called a trophotaeniae. (I had to look that word up and am still not sure that I spelled it correctly.) However it reaches from the fry to the mother, not from Mom to the young as with mammals.
New born Ameca splendens, a popular hobby goodied, are too big for most adult rainbowfish to eat! They are over 60mm (1/2 inch) long and as pretty nearly as husky as an adult platy!
For confirmation of the above and a lot more, go to your library, do a book search and get John Dawes book there (or more probably arrange for a loan from a neighboring library in the system.) If that doesn't work, write it up and submit it to the reference librarian in charge of Inter-library loans. I think it would be so cool if librarians all over the country were running around e-mailing and telephoning each other looking for Dawes' Livebearers book at the same time. :)
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