I am using about 110 GB of my hard drive on my 250GB macbook air, and I don't know why. I don't have any movies on it, and I only have about 10 minutes of home video.

Did you check the files size of each movie file? Even though you have only 15 minutes of home movies, each one may range in sizes between 40 MB to 500 MB; or perhaps even more. It depends on other parameters such as the video format or video resolution. However, I think it's best to store movies and music on an external hard drive. With today's prices, you can purchase a 2 TB drive for $70 (depending on where you shop online). For example, I couldn't understand why my Mac mini wasn't fast and peppy as when I first bought it. When I noticed how much space my music was occupying, I moved my iTunes music folder to a 2TB external hard drive. Now my Mac is running much smoother and faster now. I've had over 310 GB of music plus the space consumed by the operating system on a 500 GB drive. Hope that helps...good luck!