This document describes email size limit with attachments and why an actual email size may be larger than expected, when processing through a Cisco Email Security Appliance (ESA).
Why are mail attachments bigger than the original file?
MIME (Multipurpose Internet Mail Extensions) specification, defined in RFC 2045, lists "base64" as one of several binary-to-text encoding schemes. MIME's base64 encoding is based on that of the RFC 1421 version of privacy enhanced mail (PEM), it uses the same 64-character alphabet and encoding mechanism as PEM, and uses the "=" symbol for output padding in the same way.
MIME does not specify a fixed length for base64-encoded lines, but it does specify a maximum length of 76 characters. Additionally it specifies that any extra-alphabetic characters must be ignored by a compliant decoder, although most implementations use a CR/LF newline pair to delimit encoded lines.
Thus, the actual length of MIME-compliant base64-encoded binary data is usually about 137% of the original data length, though for very short messages the overhead can be a lot higher because of the overhead of the headers. Very roughly, the final size of base64-encoded binary data is equal to 1.37 times the original data size + 814 bytes (for headers).