I had a young guppy mom who had her first batch of fry. She had about a dozen of them (at least that I found) but remained just as fat afterwards. After a few days it was clear that she was done even though she was still very big so I let her loose in the tank. Exactly a month later she had a huge batch of fry, over 30 and when she was done she was nice and thin again. Although she definitely got bigger during the last week of the second pregnancy she stayed relatively the same from week 3 of the first pregnancy until she gave birth the second time. I suspect that the fry develop at different rates and she stayed fat because she had some beginning to develop although not yet mature enough to be born. This second batch of fry was not only much larger, but the individual fry seem to be developing at a faster rate, so maybe they were a little more mature when born and she just "held them in".