Short story: I installed Yosemite on a machine with 16GB of RAM. Without any programs open and disk cache purged Yosemite uses about 4-5 GB of RAM just for ordinary system processes. If I launch any program, let's say iTunes, it adds about 2GB of used RAM. Then I installed Windows 8.1 on the same machine. Without any launched programs Windows 8.1 always uses about 2GB. I launch Chromium and test with iTunes too. Windows uses a bit under 3GBs of RAM. Holly shit, OS X sucks at RAM management. I open more programs in Windows 8.1 and I use less RAM for the same productivity