Is there a size limit on attachments in CiviMail?
The maximum file size attachment can be set under  Misc (Undelete, PDFs, Limits, Logging, Captcha, etc.). Navigate to CiviCRM >> Administer >> System Settings >>  Misc (Undelete, PDFs, Limits, Logging, Captcha, etc.). Try after updating the maximum file size.

HTH

Pradeep