Hi @tmrrow,
Do you mean transferring files from Sharepoint on-prem to SharePoint online?
Could you provide a screenshot of your Flow configuration?
I did a test on my side, sorry that I don't have a SharePoint on-prem test environment. So I used the SharePoint onlie action throughout the testing process.
SharePoint connector does not make a significant difference between On-prem and Online, the restrictions should be the same.
I use Get file content-Create file to achieve the purpose of translating files.
Flow configuration:

Flow is successfully run, and the file is successfully created. The file size exceeds 40M.
The reason for adding Delay is that it takes time to create a file of such a large size, you can set a delay for a certain time, and then observe whether the file was created successfully.
The file was successfully created and can be opened, as below:


If you are not using this method, you can give it a try, or provide more details about your Flow configuration, we would provide a proper workaround for you.
Best Regards,
Barry