You could cut off the image fat (as you are calling it) yourself too, but that would mean that you would have to correctly position each frame, if you used a different canvas size per frame. Personally I use 1 canvas size per animation, as you can always align the animations up later with the animation center &/or the animation studio (harmonizer).
Not shure if it fits here (my weak english] but I load the image sequence as layers in gimp - cut them all in one act and then export the layers again as separate images. You need a little script for this,
you can download here. So all images are identically cutted.
This takes about 10 - 20 seconds. But as AFRLme wrote, it doesn't affect the file size as much as I thought.