How does my Molly give birth to all these babies?

Me and My dad breed fish, Mollys. He has a big fish tank that has fish in it, including some mollys, we take a female & a male out (we can tell the difference between male & female) and put them in my smaller tank. Then, when they have babies, we take the Mom and Dad into the big tank so they wont eat them. Usually we have around 10-20 babies. I heard Molly mommies can carry more than one batch of babies at a time. Yesterday, my Momma molly gave birth to 40+ babies. As usual, i took them out. But I didnt see my momma molly get any bigger. She stayed her size. They babies are tiny, but not itty bitty. How did she give birth to all of them? How did she carry so many fish? Please, no mean answers. Thanks:)