How do we know thinness drives sales? If thinner devices, which also are faster with more features etc sell more than the previous edition, the cause is quite d

How do we know thinness drives sales? If thinner devices, which also are faster with more features etc sell more than the previous edition, the cause is quite difficult to pinpoint, surely? The fact that devices get thinner and devices are sold in ever more numbers/profitably does not mean the device dimensions are the cause.