Mirror iPhone 11 sizing issues

When I mirror a frame (iPhone 11 size: 375*812) imported from Figma to Principle, with the Principle Mirror app on an iPhone 11, the frame shows up smaller than what it is supposed to be. I get a black surrounding border on the sides and top and bottom of the screen. In other words, it doesn’t ‘fill’ the whole phone screen. Is that normal? How could I fix that?
When I use “Figma Mirror” straight from Figma, it works perfectly, filling in the whole of the iPhone 11’s screen. Not when using “Principle Mirror”.
Would love some help with this. Thanks!