Uploading the zipped file to dropbox and unzipping it inside would save a lot of time and traffic for users. Some of the web services already exist for that (like this one http://www.quora.com/Is-there-a-way-to-decompress-a-zip-file-inside-Dropbox), but as they use their own server capacities, they cost money. Unzipping inside the dropbox may be an additional feature inside the dropbox itself. I would even pay for that a little xtra.