Emulating a setup similar to yours, the OSX machine infront of me sits a 2.8GB active, of which a little less than 800MB are taken by Terminal.app due to me having activated infinite scrollback and having two rails processes being hit 8 hours a day since like forever as I develop. So it's really closer to 2GB and I didn't even try hard. This includes mds+mdworker (the indexer) which clocks in at 200MB. Normally I also have LibreOffice, Pixelmator, iCal, iTunes, Reeder, VMWare Fusion, which makes it balloon to 3.1GB and up as I open more documents/VMs.

OSX is not doing anything "wrong".