Why would a guppy only produce two fry in four days despite being very large?

I keep platies, which are closely related to guppies, and their breeding habits can be bizarre to say the least. If I were you, I wouldn't be that worried. I have had platies swell to enormous proportions, drop about ten biggish fry and then miraculously shrink back. It might be a hormone thing. I have had my girl Ranger throw six fry, remain huge, wait another two days and then throw another twenty eight. I have seen livebearers - mostly guppies - that are obese so that they look like they are pregnant when they're not. (Males). Fish can get fat and its possible that yours has been overfed. Also, livebearers eat their fry. If your female was really big, then she dropped the fry and shrank back down but you only saw two, it's likely that she or her tankmates ate the fry. When they are about to have babies a hormone stops them eating them as soon as they are born, but after about twelve hours it goes away and they will eat their fry. Most fish will eat small fry. If she is having trouble, looks uncomfortable or stays at that size without having more fry, ask the people in your local pet shop. They generally know a lot.