Why is my fish such a pig?

I have 3 male platies and one female platy in my 10 gallon cycled and heated tank. There's a boy betta, a mystery snail, and four ghost shrimp in there, too. My female platy eats the most. She always races the guy fish for the bloodworms, steals food out of mouths, pushes for food, etc. I just bought algae wafers for my snail and shrimp today [the fish have already eaten] and sank one wafer to the bottom. I left for 2 hours, returned, and found my female platy as big as my mystery snail. Will she be okay? She looks pretty full...br
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Oh, and why does my female platy prefer to hang with my biggest male platy? The biggest male is also as big as my betta and pushes around the smaller males. The smaller males just hand by themselves, mostly... It saddens me. Is this bad? If I return the big male, where do I take him? All the local fish stores are marine only. The others [petsmart, petco] wont take fish unless you have a receipt. Ive lost mine, plus ive had these fish for months.