Hi gfreemantle
Thank you for using the Xerox forum.
If you have not already resolved this issue, we suggest you search our online knowledgebase for this product
Since you are not having an issue from Windows OS then you know the tray setting on the printer is correct.
The difference is the application page setup and the driver settings.
Please review the print driver paper size settings and the page and print properties in the application to ensure they are correct.
Alternatively, you can visit our Contact Us page for other support options. Our product trained experts can guide you with specific Mac setup options.