I was running through my settings when i stumbled upon this. I'm curious what is actually stored inside iCloud (by Dropbox), because the files inside Dropbox are obviously not (my Dropbox folder is much bigger than iCloud capacity)
Definitely it's not the data you put on dropbox cloud. I think this old answer hints about your question. I know what time machine back up is but I think this service might later be included in iCloud backups too.
As for iOS, ios backup
details explain what it stores.