Why did this GIF file get so big?

[Ofnuts]

1) Possibly because the GIF was "optimized", with many frames being just the pixels that change from the previous frame, and thus being small. When you break it up into individual images, you generate 612 individual complete frames. For instance, if you have an optimized animation with one pixel moving over a 200x200 background, you have a first frame which is the initial state, and all other frames are 1x2 frames (or 2x2 frame for diagonal moves) where one pixel restores the background and one pixel paints the new position. When you break it up, you go from (1×200x200)+(199×2×2)=1196 pixels encoded in the optimized version to (200×200x200)=8000000 pixels (almost 7000x bigger...).

2) Did you re-optimize afterwards?