For the sake of accuracy the size of gnome-icon-theme.tcz in repo is stated as 7.63 MB.
However, of course this is still more than enough to bother.
25.2MB is the size reported by emelFM2 once decompressed.
Makes sense...
What I would try in such a case is making a customized "fat-free" extension which contains only the files really required.
My understanding is that this is synonymous with the action of my second bullet point.
Ok, seems I hadn't exactly understood then.
The more conservative and safe approach would be to remove everything redundant from a package.
What I would try in specific case would be to copy all the files
not ending in .png plus the required icons, all with their
full path.
I could imagine proper use of gtk-update-icon-cache might also be indicated, after installing the icons.