I am finding that when I look through the SD card on Astro that many of the applications I downloaded and uninstalled still leave a directory - sometimes full of stuff.
So you may be uninstalling stuff but it's not completely uninstalling and therefore taking up valuable room on your card.
I connected my Droid via USB and opened the folder and found that there were a bunch of directories that needed deleting.
So you may want to do some housecleaning if you're comfortable working with computers and the Droid.
An application that may be useful is one that cleans your SD drive of any files that are no longer connected to their applications.