There is a limit on the total size of the message (message + attachments) transmitted. The limit is 50Mb and is calculated as follows:
If the value of [rcpt to] * [message size] exceeds 50Mb, the transmission of the message will be canceled.
This limit is to maintain the performance of the mail server. We have found many clients who send very large messages with very large attachments to a lot of recipients and this ends up slowing down the mail server affecting the rest of the users on the server. If you need to send a larger file to someone, just upload the file to your website and send them the URL so they can download it.