DPI?

It's set to 72 on both of them. The only thing I can think of is that the bigger one has significantly more images and layers, but the .fla file is still only like 1.1MB, so I don't understand why the gif would be three times that file size.

so the end file is a .gif?

gifs aren't like fla's, with objects and timing and movement information (your browser renders everything, the user pretty much only has to download the compressed library objects). Think of a gif as a file composed of tens of jpegs with timing information (the developer renders everything, and the user has to download all of it for it to display).

.. that didn't come out too right:
gif: flip-book animation
flash: click something with your mouse and drag it across the screen animation

If nothing changes on the next frame it'll just use the same "image" as the one previously and wham you save that in file size. But if it's got to change, wham that's another image the file has to store, and at 20fps or even 12fps that can get hefty really fast.

So lots of movement in a gif = lots more images to store
whereas minimal movement = lots of repeated images (which are in fact the same 5k of data repeated by the gif's timer) and you end up saving mucho space.

Ok, that makes a lot of sense. The larger one has four images that move slowly across the stage, so every frame is different.