why?

all fish, like all people are not created equal. they may grow. they may not. how many wee ones do you have? how big is their grow out tank? what are you feeding?
 
They all live in a 20 high, with 3 adult guppies, a female betta, and 6 ghost shrimp. I feed flakes 5 times a week 1x per day and frozen bloodworms the other 2 days also once a day. I don't believe feeding is the problem because they all plump up fairly well from feeding, even the little ones. And as I said before there are 5 teeny ones. Also I should add that there are 9 that are growing well. Down quite a bit from the 72 I had when they were first born but that's why my betta is there.

Oh by the way what could I get to help my betta keep the numbers down? I know my tank is already overstocked but all of these babies except 3 will be moved to new homes in the next few weeks, and I don't want to keep having to deal with finding homes for fish, the only LFS near me is not a good one unfortunatly and I do not want to give them any of my fish. So is there anything I could add to the tank that will get along well with the female betta?
 
Could the fry be from different "litters"? That would explain the size difference.

Sounds like your betta is doing a great job with fry control. Any fish will eat fry, if they can fit in their mouths, but there will probably always be a few that make it. Can you seperate the male and female guppies so they stop mating?
 
Seperating the males and females is a good idea. Just know that livebearer females have the ability to trap sperm and essentially fertilize themselves at least once down the road. I've had platies and mollies give me fry 2 months after I bought them out of a mixed tank.
 
Guppies can give birth up to 6 months after they were first fertilised by the male. The growth rate of fry does vary enormously. I have fry that can be twice as big as their siblings of the same batch at 8 weeks old. As long as the majority of them are growing well I wouldnt worry. Seperating out males is not just good to stop them mating but also to stop them harassing the females and causing stress. In the long run it prevents less disease. I find the fry grow much quicker if put in a grow out tank that gives them plenty of room to swim about in.
 
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