I suspect that the email service of the RECIPIENT of your email is presenting the size limit problem. Your ability to send an attachment to someone is limited to the SMALLER of the limits imposed by the sender's server and the receiver's server.
iCloud caps attachments to 20 Megabytes:
iCloud: Mail specifications
10 Megabytes is the general rule of thumb for email attachments (However, you should leave room for encoding overhead). If your file is close to this, try zipping the file, or using a service like fileChute, DropBox, or good old fashioned sneakerNet.