It’s because your whole content has a height of only 847px which you can find using inspect element by hovering the <body> tag.
Whereas, iPad pro and Kindle has a min-height of 1200px plus, you can see that in the screenshot you provided. This is the reason for the whitespace, since the body has covered only 840px out of 1200px approx, rest of the area is left out as whitespace.