[dmgd]

both manners and the concept of "volume" seem to have escaped you.

using figured from ohwp's response above:

  iPad    : 6186 cm3
  iPad 2  : 3944 cm3 = saving of 2242cm3
  iPad 3  : 4213 cm3
  iPad 4  : 4213 cm3
  iPad Air: 3060 cm3 = saving of 884cm3 (vs smallest other iPad, the iPad 2)

[pygy_]

In both cases a diminution of ~30% (35 and 27, respectively).