I have been using Windows 7 on an external H/D on my iMac & MacBook run under Parallels.
I removed it from my iMac about 6 months ago but continued with the MacBook because my software accounts package was under Windows and a couple of other Windows only programs.
Having retired I no longer require the accounts package and have found alternative software for the remaining programs, so removed Parallels which encompasses Windows from the MacBook.
It released 60gig of space, when you consider what's left contains, Lion, the usual default programs that come with a Mac iTunes, Garage Band and the like, also CS4, Lightroom 4, Aperture 3, Office for Mac, a number of Nik plug-ins, sundry minor programs, and of course some data (not many images these are kept on a external drive) all in less space 50gig than the 70gig Parallels/Windows occupied.
The Windows programs included just Account software, Elements 7 (no Plug-ins) GoldWave, PhototoEx, but I beleive this was on the external H/D for Windows/Parallels to use.
I knew Windows was much hungrier for space than Lion but this amount I find amazing.