When simply right clicking for the properties on the XP Windows folder here with only a few programs besides video and sound installed 2.93gb for size and 2.71gb for size on drive is seen. The newer version after this many years is obvoiusly going to be larger as XP is larger then previous versions of Windows itself.
9x-ME, NT was never anywhere near as large as XP. And what is the main difference between 16, 32, and 64bit? Not simply memory capacity but seeing larger partitions. With the new 1tb drives now seen and larger to come the 64bit structure and of course all the newer features(eye candy) that bloat up the new version.