Stephen Hackett at 512 Pixels has a great writeup on how the iPad has changed in 5 years — and how much it hasn't. The most striking thing to me is that it's gone from 1/2 inch thick to 1/4 inch. As he points out, "the entire iPad Air 2 is as thick as the sidewall on the original iPad." (Referring to the gently tapered profile of the original iPad.)