Typical size of an Ubuntu system: ~4 GB, including filesystem inefficiencies. You can usually fit an archived backup onto a DVD without even compressing it.
Typical size of a Windows 7 system: more like 12GB when you first install it. And it grows as you add and remove stuff. And as temp and log files accumulate, and patch uninstallers are stored, and System Restore snapshots are automatically allocated, and...
What can I do about this?
(Rationale: smaller backups, hard disk space savings on older machines with IDE drives, and not polluting the various caches with junk.)