I think this is kind of normal. My PCF/PTF's were doing something similar. I was worried at one point they were getting too fat, but then they would slim down after a few days. I think they have slow metabolisms and after a healthy feeding they may take longer to digest and look a little plump. After the food works its way through their system they slim down until the next feeding. This is all just a theory I have formed from watching mine.

Although I have noticed the fluctuations are not as dramatic when they are fed mainly fruit flies compared to a cricket. In fact just today Atwater grabbed a fly that snuck in while was changing their water dish, and it fattened him up quite a bit. I'm sure he'll slim down once it gets through his system.