Try Changing Gmail’s Security Settings There might be some settings conflicts at the Gmail’s end, fixing them may give the. Once done this open the App and refresh the page. I dont, however, know if there is any limit to the size of individual messages with Gmail. In Settings, locate Passwords & Accounts in the menu.
Scroll down until you see the Check mail from other account: section and click on Add mail account. To import OLM files to Gmail, OLM File Converter is a great option.
Then, click on the Settings gear icon and go to Sell all settings > Accounts and Import. try to Import your OLM database file of Mac Outlook to Gmail.