Amazon Appstore for Android has a nasty habid of leaving old files around. This can cause major problems when your SD card fills up and you don't discover it until you're trying to snap a photo. What happens is Amazon downloads the APK file to the SD card and then installs it. That part is fine and necessary, in fact. The problem is the file never gets cleaned up. I can't think of any good reason for that. Maybe there's an argument for leaving it for a brief time, just in case you re-install the app, but not forever.
The workaround is to nuke the cache folder. On my DroidX, it's in /sdcard/Android/data/com.amazon.venezia/cache/. Just delete everything in there.
The correct solution, of course, is for Amazon to clean up its own temporary files.