Probably partially related to new RAM compression. If the space gained resemble the amount of RAM you have this might be the case.
The reduction might also be related to the sleepimage file which is a hidden file that is usually the same size as the amount of RAM you have. If they compress it or only save what is necessary that would easily "save" 14 GB on my system (mine is 2GB on Mavericks with 16GB RAM, I don't know what it was in Mountain Lion).
Or just some code cleaning in the OS, but 15+ GB gained is a bit high to be explained by just that. Wiping various caches/page files/etc. List goes on...
Just curious, how much RAM does everyone have?