Standard warning: this post involves deleting data, potentially lots of it. Look twice before you leap and be sure you have backups, just in case.
For a long time, buying an HD iTunes Store film meant that iTunes automatically downloaded both the SD and HD versions. It used these SD versions to offer the option for loading just SD copies onto devices in order to save space.
At some point, iTunes stopped automatically downloading the SD copy of an HD film, but it kept all those existing SD versions around. I never really used those SD versions so I didn't pay much attention, but I finally decided to see how much space they were talking up. Turns out it's a lot.
If you've been buying a lot of HD films from iTunes all this time, and you're interested in saving some space, it might be worth your while to delve into your iTunes Media/Movies folder. By default, it's in your Home folder at ~/Music/iTunes Media/.
Each movie gets its own folder containing both the SD (where applicable) and HD versions. My SD films are around 1-2GB each, and I simply started deleting those versions. iTunes doesn't seem to mind much, though I also haven't tested what happens when you enable the SD-only transfer option since I just stick with the HD versions.
If you try anything, be sure to look twice before you leap and make sure you have backups just in case. As you can see, I saved a good chunk of space doing this. I hope it can help you too.