A recent Apple Support Note warns of an iTunes cache folder that can swallow your hard drive. Even better, the size of the cache can be bigger than the sum of your photos.
The problem is really about syncing photos with your iPhone or other iOS device, according to the note.
When you use iTunes to sync photos to iPhone, iPad, or iPod, iTunes creates a folder called "iPod Photo Cache" in the top level of the folder you selected for your photos. Picking another folder to sync does not erase the previous iPod Photo Cache folder. Depending on how many photos are being synced, your hard drive could fill up.
The fix is to dig down into a Library package and pull out the folder.
There's a bit of additional information at the bottom of the note that says that the iPod Photo Cache folder "can be larger than the actual size of photos being synced."
Sweet! It must be a feature.