Unclouded, by Christian Göllner, works with Dropbox and Google Drive, and can be used to quickly analyze how your space is used in either service.
Right now, the app is in beta, which means that you will first have to visit its Google+ community, and agree to become a beta tester, which will then allow you to download the app.
Right away, you can swipe in from the left to see all sorts of options. The first thing you’ll want to do is add an account. Once you’ve added an account, you can view your quota usage, as well as using a file browser to see how large directories and things are in their folder hierarchy (though you’ll have to upgrade in order to be able to delete and manage stuff right through the file browser). You’re also able to view files by category or last modified, or even see where you have duplicate files.
The folks over at CNet have done an excellent writeup of the full features of the app as well as how to use it, so if Unclouded seams like your cup of tea, head on over to see how it works and give it a try.