Platies that are fully mature and not past their prime can often deliver 40 or 50 fry at a time while mollies of the same age and condition can deliver over 50 regularly. They are different fish species and do not have the same genetics so I have no idea why they would deliver the same number. (Yes I restated the problm backwards.) Swordtails can easily deliver as many as 150 fry from a mature female in her prime and they are closer to platies than the mollies are. Swords are close enough to platies to actually breed with them successfully and have fertile offspring. In a typical hobbyist's tank, many or most of the fry that are born will not survive the initial predation, so the mollies may just be better predators than the platies in your tank.
You can go a very long time between cleanings if you have the right filtration and enough plants to remove all of the contaminants that would accumulate. For an NPT set up by Walstad's method, the recommendation for water changes is about 6 months, but most of us will not tolerate enough vigorously growing plants to be able to call ours a natural planted tank. I have one set up as if it was going to be run like an NPT but only go about a month between changes on that tank. The tank size doesn't matter, the more fish you have the shorter the time between changes and the more plants the longer you can go. A ratio of fish load, plant growth rate and water volume is the only way that water volume plays into the equation. For the same relative fish load, the time would be about the same.
