I noticed some skin files included in v3 RC are of an enormous size. Most of those extra bit seem to come from splash screen, unknown artist or no artwork images included in those skins. The size wouldn't matter if those are available from a link on the skin's post, but when it's part of the official release file it would be better with optimized size as that would allow more skins to be included in place of some high-quality splash screen images.
There are many on/off-line tools to optimize images. Some are lossy and others lossless. Among them I found tinyPNG.com quite effective (of course no personal affiliation to the site). I also used it to reduce a screenshot of skin image on skin posts because imgur.com changes a png file to a high-compressed jpg file (really poor quality) if uploaded file size is over 1 MB. The screenshot of my Pastels skin on the skin's post was originally 2.5 MB but after optimization from tinyPNG it had reduced to 0.5 MB with almost no visible differences. Hope this helps.