Hi Stuart,
Thank You for using syncfusion products
We have checked your query. We have found that UWP sample background and SfPopup is not get enlarged is a defect in our side. As we have mentioned SfPopup control is in preview state. Sfpopup control full release is on the next release which is scheduled to be rolled out by the end of march and the upgraded improvements and this issue fix will be available in our next release.
Regards,
Suriya K