And why is this important? If you are concerned about space usage, consider the SAS system option COMPRESS=YES, however the results will be data-dependent. And consider that character versus numeric data stored by each could have an impact on size.
Sas has to fit database usage requirements like:
-maintain dataset header information,
-maintain variable metadata (formats, etc),
-allow instantaneous direct access to any record,
-use fixed length storage (unless you request compression)
-allow hooks for aging, integrity constraints, index usage
Excel doesn't do any of that, it just stores in whatever (probably somewhat compressed) format it sees fit that opens the file in a quick fashion. None of the constraints above apply (but others do, like loading the whole file in one go), as the usage is totally different.