If you right-click an image and click 'view image info' (or properties), you can see exactly how big it is.
(Though, sometimes the file size doesn't show up properly. It works better if you copy+paste the img url in a new tab and check the properties again.)
The animation you have in your signature right now is; 1,015,336 bytes, out of the allowed 100,000.
As I mentioned, animations are often quite large, due to the frames.