The Nokia N900 comes with a root filesystem with a minuscule 256MB. Nokia and the Maemo community have done a good job of making Maemo work with this through the liberal use of symlinks, but a limitation is still a limitation, and Nokia's still trying to get it right.
This morning I had about 27MB of free space, or about 10%, free. This hasn't been a big problem before, but today the first Maemo update after I received my device was released, and it need about 14MB to hold the downloaded files, and would need about 17MB of space extra after extraction. After combing my filesystem, I realized that I had over 6MB of Maemo themes installed to the root filesystem.