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

The Helix specs are ...:

480 x 800 = 384,000 total pixels
3" x 4.6" = 13.8" square inches (unverified)

384,000 divided by 13.8 = 27,826 pixels density per square inch ...

(If my calculations are correct on the Helix) ...

So the Helix total resolution is almost as high as the 798 ...