Destination: G Suite or Exchange/Office 365
When migrating mail into G Suite/Office 365, the error message "Message is too large and attachments cannot be removed" is returned for some items. It occurs when a mail item exceeds the default 50MB size limit in Gmail/365.
You can edit the Max Message Size in the Destination Platform Migration Settings accordingly (this is available as Message size is customizable in Exchange)
G Suite destinations:
The Gmail API now supports emails up to a max size of 50MB. If the message itself (not including attachments) is larger than 50MB, these messages cannot be imported to Gmail.