SMTP is for 'Simple Mail Transfer Protocol' and, when initially released, was just for text. When the world decided it wanted to also send attachments, the system was adjusted to package attachments into a format that 'appeared' as text to the process, yet could be reconstructed on the receiving side. If you press Cntl and 'u' to a sent message that had an attachment, you will see an example of this. That repackaging increases the eventual size. My explanation, I'm sure, isn't 100% accurate in wording or sequence of events, but repackaging is what caused that size increase.

Based on the information provided by david, I found the following link: Base64 https://en.wikipedia.org/wiki/Base64

Quotation from the above link:

the actual length of MIME-compliant Base64-encoded binary data is usually about 137% of the original data length (4⁄3×78⁄76)

21.5MB * 1.37 = 29.455MB, which matches the value you saw.

Encoding attachments adds about 33% to the message size, so a message with a 21 MB attachment results in a message size of about 28 MB. Some services specify a maximum message size, others a maximum attachment size.