I've fiddled around with different images after reading Yorik's comment
“. . . AFIAK gifs do not store the whole image per-frame, but rather sub-frames that change.”
It does seem like that's the case, since testing with this gif gives a much smaller difference between images saved with and without transparency (source):
- With transparency: 531 bytes
- Without transparency: 502 bytes
That's a saving of ~5%, which is much less than the ~50% savings that we get with the image in my original question.
So it appears that transparency allows for some optimization related to changes in the image. I can guess how, but I'd rather hear from someone with a deeper understanding of the gif format:
Why does enabling transparency in an animated gif reduce its file size?
I've updated the title to reflect this update, but the original question is included below.
In Photoshop's "Save for Web", why are animated gifs twize as large with transparency disabled?
I have a gif that I want to modify in Photoshop:
When I'm ready to save, I use "Save for Web" and look for ways to optimise it. These are the settings that are loaded automatically. The estimated file size is 1.871M.
I uncheck "Transparency" since there is none in the image, thinking that this might reduce the size. But instead, the size is effectively doubled. The new estimated file size is 3.739M.
So what I'd like to know is... why?
Also, I'm using CS6 if that matters.