I have decided to turn off FileVault on my PowerBook. When FileVault is enabled, your home directory is copied into an encrypted disk image, which is mounted when you log in. This disk image is supposed to be a sparse disk image, which is supposed to increase in size to fit the contents of the image. The size of the disk images is supposed to decrease when files are deleted.
It looks like Apple's implementation is not that good. When you create a new file, the disk image size is increased. This happens, even though a larger file was just deleted. The only time that the disk image is compressed is when the computer is shut down, and I hardly ever shutdown my laptop, but instead I just put it to sleep.