I think you've covered most of the reasons. Compression probably also reduces the actual storage size of files. They are probably also using some of their own s

I think you've covered most of the reasons. Compression probably also reduces the actual storage size of files. They are probably also using some of their own storage outside of Amazon. Rarely accessed files could be moved to slower, cheaper storage facilities. But you wouldn't get the full answer without access to internal Dropbox documents.