Each product require 20-200MB disk space. But the installation requires a ~1.5GB base disk space regardless of what you install. What does it use that 1.5GB disk space for?
It would appear that Microsoft is conservative in regards to the amount of disk space and purposely overestimates the amount you'll need. Taken from a Microsoft TechNet Article about the Office 2010 System Requirements underneath What About Disk Space?: