Pixel densities...

The different size screens have different pixel densities.
The 798 has 31,170.5 pixels per square inch.
The 898 has 17,533.4 pixels per square inch
The 998 has 12,755.7 pixels per square inch
The 1198 has 8,858.2 pixels per square inch

In the world of sonar displays, the 798 is HD.