so I hit "get info" on all the folders in my home folder... add them all up, and I get 55. Istat says I have 70 GB taken up on my computer. Where's the extra 15 GB?
Uh, in your root Applications, System, and Library folders. "Home Folder" is not the top level folder, your hard drive (usually called Macintosh HD) is.
Your home folder is actually several folder levels down the hierarchy.
My Applications, Library, and System folders combined contain 42 GB of stuff!