That's not because of how the memory is managed that nix or XP are bloater. XP is (nearly) bloated and OEM XP are certainly. On the linux side, OpenSuSe is very heavy (how the hell can I avoid having all the mono stuff ?) but nor Slackware nor most BSD are.
Err... I think you misread my post. I'm not saying that Vista or *nix systems are bloat - I'm saying they're smart about memory management.
Your comment about XP and OEM XP... there's no difference between the two! They're the exact same underlying system. The only difference between the two is licencing. XP comes with a small handful of tools that helps you maintain your system (there are some better third party tools, but let's not go there). I really don't see what's so "bloat" about Windows, nor have I ever understood the definition of "bloat", since people keep talking about different things (memory usage and management, bundled programs/files... etc).
OpenSuSe is only "very heavy" because of all the additional programs that they bundle with it. Pretty much all BSD systems I've tried so far drop you into a console on first boot.