email appliance - tls/spx encrypt greatly increases file size

Hi.  I am running a Sophos Email Appliance v.3.6.3.0.  We use encryption for sensitive messages.  One of my users complained his emailed spreadsheet was blocked at the other end due to size (>10MB).  When I checked the logs, it showed original size=5093341, 40secs later (after encryption)=10535571.  This is a substantial increase.  Does this sound normal?  I guess I need to do some more reading to better understand the process from point A to point B.

Thanks.