Win 7 Home Premium 64 bit Install Size

I installed it and it took 13gb. Did you possibly do an update? If so, your old OS is in a folder called windows.old, and you can delete it.

No, clean install...says 20 gig here but I figure there has to be a ton of stuff that is safe to delete...

http://windows.microsoft.com/systemrequirements

Look at control panel/programs and features. Uninstall anything that you don't need.
When satisfied, empty the recycle bin. You might also check how much space is reserved for the page file.
I use Windows security essentials. It is free, takes little room, and is completely unobtrusive.