The biggest iOS device of all time was the iPad 3G at 13.4mm thick and 730 grams. The iPad Air LTE is 7.5mm thick and 478 grams. 7.5mm is only thicker than 4th and 5th generation iPod Touches (7.2 and 6.1mm) and the original mini (7.2). The iPhone 5/5S is 7.6mm thick. The retina iPad mini has “bloated” to the same 7.5mm as the Air, adding to their similarity.
Thickness isn’t the entire game of course. I found the iPad 2 to be “sharp” in my hands because of the shape. I actually felt it was a little uncomfortable compared to the iPad 1, which I always used caseless, but the iPad 2 I usually had smart-cover propped. Despite the iPad 3/4 being significantly lighter than the iPad 1, I found the 1 was more comfortable to hold too.
I have a spreadsheet of iOS dimensions and weight, and a calculated column of their “density” – their weight per volume. The original iPad with and without 3G are the least dense iOS devices Apple ever made at 1.10 g/cm3 and 1.18g/cm3. There’s a huge jump to the iPad 2 at 1.5. 1.5X is where all of the iPads are now, except the minis which tip 1.6X for retina and LTE models.
Density has a weird way of playing with our perceptions. A 10 lb weight is not much useful for a workout, and feels light and insignificant. A stack of 2 retina MacBook Pros at 9 lbs that you pick up with BOTH hands feels like it weighs a ton. (Tip: don’t stack laptops).