Fish do need a lot less food than you'd think. Animals like us and our most common pets like cats and dogs use a huge amount of our food to keep ourselves warm. Fish don't keep themselves warm, they rely on their surroundings to do that, so a mouse sized fish doesn't need nearly so much food a mouse. We tend to think in terms of what we'd need, and end up over feeding.
As a very very rough guidleine, one fish needs as much food as the size of its eye. This really only applies to flake food, or maybe pellets, it doesn't really work with bloodworm or algae.
Fish always act as though they are starving. They don't have a mechanism that tells them they are full, and if you let them they could eat themselved to death.
When you are fish-in cycling like JokerFish, or still culturing your bacteria from a media donation like Billy, you do need to feed a bit less than that so that the ammonia level doesn't go up too quickly or the transplanted bacteria don't get too stressed. So either half the amount you should be feeding every day or the normal amount every other day.
JF - breaking the flakes up if they are big can also help, and scatter the bits. It will take the big molly longer to eat a whole flake that way, and the little one stands more of a chance. Did you know that mollies also like vegetables? Microwave a couple of peas in a bit of water, or save some from your meal, pop the insides out of the skin and chop them up till the bits are small enough for the small mollies mouth. One pea per fish. If there's any left on the bottom after a couple of hours, remove it.
Scooter User - I go by 'feed as much food that will be eaten in 2 minutes'. Though that does depend on what fish you have as some are very slow to realise there's food in the tank.