(Specs taken from DisplayMate's review of the screens. I am not allowed to post links.)
Ipad Air weighs 1 pound with a screen area of 45.16 square inches. That's .0221 pounds per square inch.
Kindle fire HDX 8.9 weighs .825 pounds with a screen area of 35.6 inches. That's .0232 pounds per square inch.
The Kindle is no lighter than the Ipad Air, just a little smaller. The real question is why the other android tablets are still so heavy.