I notice that the Android Dropbox app, when uploading a file to the Dropbox server, first copies the source file to a cached file. Then it uploads the cached file to the Dropbox server.
Why? I don’t know for certain. Maybe the app developers wanted to ensure that the source file isn’t altered during the upload process.
The cached file consumes precious storage space on your phone.
Regain lost storage space
To regain that space, delete the Dropbox cache. You can do this from within either the Dropbox app or Android’s settings (Applications, Application Manager, scroll down to Dropbox and press the Clear Cache button).