okay, that makes sense why its so large filesize wise
this is what you need to do... get rid of that black mask, it has alot of edges on it, and that takes up alot of space, if its just solid colors, it will shatter the skin's size, so just delete that black mask, and the blockier the colors you have, the lower the filesize (well.. the larger the blocks), soft edges increase filesize because that just adds that many more colors it has to save
That may be something... But still... I can really not see how that file could have been 3MB... I've done skins with more colors, details, and at 2048x2048, saving it as 100% quality jpeg, and gave no more than 1MB... heh.
The black mask should reduce file size, not increase it. The mask actually reduces the number of edges around your edited work and as well it provides a large block of solid colour instead of hundreds of shades of grey present on the unmappable areas of the skin. When you paint or add objects to your skin you are creating many edges, the mask can tidy up the borders of your work effectively reducing the edged area. Having many colours or hues around the actual edge of the JPG can also lead to a slightly larger file, but if the entire perimeter is one solid colour (masked) there should be a saving of several KBs. It won't hurt to try though, paint your skin and save it. Now add a mask to the same skin and save that. If the masked version is smaller then it was probably worth using the mask.