This is probably because you did not officially format the drive. If you do a clean install on top of a previous OS with actually formatting, it will stick everything from your old OS in the Windows.old
folder and take up tons of space.
A while back, I experimented with clean installs of the different editions of Windows 7 to see if higher editions would take up more drive space. See here
You can use Disk Space Reporter
to tell you what is using up your hard drive space.
A clean install should take up less than 10GB. I'll bet all that extra is in the Windows.old folder, which you can safely delete if you don't have any data you need from your old install.