Indeed. The fact that this is specifically about the 5s and the recent iPad tells me that part of this is probably the speed of the A7. If it's faster to open apps, browse the web, etc, you tend to do more of it. That makes the device more useful to users and reduces friction, which leads to high usage.

In short: They use more data because they're better devices.

But there are also probably some of the other variables mentioned by the parent. They just apply to all iOS devices equally, not just those models.