Mutt doesn't set a limit, but is affected by the limit of the mail service being used. In this case EXIM.
For EXIM you can increase the limit like so...
- Log in WHM.
- Go to Home » Service Configuration » Exim Configuration Manager.
- Select Advanced Editor.
- Change message_size_limit to 200M (it was 100M).
- Save.
After these changes my backup worked.