Game saving random amounts of storage space/ game lagging

I saved my game last night and when it showed my world's file size it was at 708 mb. Today when I got on it said i was at 1.4 gb, then after playing for a few hours I saved and quit and my file size was at 804 mb and now looking at my world again it says Im at 698 mb. I notice when the file size is larger, the game lags insanely and when it gets smaller it runs a little smoother but still lags at times.