Blue Alligator Support Forums

The standard styles of product thumbnails (selectable via the button on the bottom-right of your screen) were all designed for the original screen size, though due to Apple's scaling mechanisms at the time they also displayed to scale on the iPad Mini's. Unfortunately Apple changed their scaling mechanisms a few iOS versions ago though so these standard styles aren't automatically scaled up for the larger iPads.
However, it is possible to design custom thumbnail layouts through the cloud account and these can be configured on a screen-size basis, so through that method larger thumbnails for the 12.9" iPads can be utilised. If you're happy for me to pass your information onto my colleagues, I can have them get in touch with further details.