When you digitally sign or encrypt a message, it is stored as MIME instead of MAPI, causing all attachments to be stored in base-64 encoding. The attachment size will increase by 33.33%. This increase continues even if you do not use digital signatures or encryption in later replies and forwards.
When you digitally sign and encrypt a message, Outlook signs the message causing a base-64 increase of 33.33%. It then encrypts the message causing a base-64 increase of 33.33%, resulting in a message size that is 16/9 larger (4/3 x 4/3).
If you use RTF (Rich Text Format), the message is stored together with a HTML formatted body PLUS a RTF formatted body, additionally increasing the message size.
When you use plain text, the message size grows less.
When you digitally sign the message, additional overhead includes the certificate chain, and the S/MIME information.
When you use encryption, additional overhead includes the lockboxes for each person for whom the message is being encrypted.