By default, mailboxes are set up with the message size limit of 50 MB for incoming and outgoing messages.
Due to message encoding used to transfer the message through the Internet, the size of the message can grow substantially. The "encoded message size" is the size of the resulting email message with all of the attachments added and encoded using MIME (http://en.wikipedia.org/wiki/MIME). This encoded size is the actual size of the message as it travels over the Internet and is always larger than the raw size because of the MIME overhead and because binary attachments are generally encoded using base64 encoding. Base64-encoded files are usually about 137% the size of the original files.
Additionally, the encoded message contains all of the metadata about the message — and this information grows as the message travels from the sender to the recipient.