Are you deliberately being silly or is this an actual argument?
The latter. You're misstating the relationship of pixel density to display size. That's the only point I'm making.
Screen size and resolution are the only to things that matter.
Neither one
determines pixel density. In fact, of the three, the only two you have control over are pixel size and the number you put together (resolution).
Of course the 7 inch screen at a resolution of 1920x1080 will require more (and smaller) pixels than a 42" screen at the same resolution. No kidding. The fact remains that the decision to go with a smaller screen is what made it NECESSARY to pack in more (and smaller) pixels to be able to achieve the 1920x1080 resolution.
It would need smaller pixels, yes. No one's disputing any of that (edit: except that the 7" and 42" would need the SAME number of pixels, just different sizes). It's just that you told jsw he was wrong when he wasn't. That's all.
They decided on 4:3 so 1024x768 was the only option on a 7.9 inch device.
It doesn't work like that. You can't will pixels of a certain size into existence. They wanted to make a tablet in the 8" ballpark. They decided to go with 1024x768. At
that point, they looked at manufacturing processes and identified their 163ppi IPS process. It is
that which then determined that their finished display would be 7.85" in size, and they designed around that.
They chose screen size and resolution, ppi is just a statistic based on those two decisions.
No, it's not a statistic based on display size and resolution. It's a statistic based on pixel size. It's not an abstract figure. It's a very physical reality.
You can't design a display and size it down to the hundredths of an inch and then assume everything magically fits. You are constrained by physical processes. You pick a general target size, determine the display resolution, and then go shopping for pixels. If they had identified a hypothetical 170ppi process to manufacture the iPad mini, it'd be a 7.52" display. If they'd picked a hypothetical 160ppi process, it'd be a 7.98" display.