Why is my startup disk so full?

I've been getting messages recently that "My startup disk is almost full," and I'm quite sure I haven't used the full 250GB of my Macbook's hard drive. Using "Get Info" on Macintosh HD says that I'm using around 245GB (aka 5GB from being full), but I have no idea what could be using that much space. My home directory (with Photos, Movies, Downloads, etc.) is only around 34GB. Applications? only 11.

In fact, I looked into it using a disk sweeping application, and it profiled my hard drive as only having 65.1GB in total! Where are the other ~200GB? What's filling them?

(I did search using Finder for files over 1GB, and there were only a few, so I don't think the problem is one massive file).