> a 21 and 27 inch iMac are still both iMacs
The 6 and 6+ are still both iPhone 6s, the only difference beside size is extremely minor (optical stabilization), not entirely different products.
> Same for the MBA and MBP, those are simply size/feature differences, not entirely different products.
See above.
> Would you also then break out each and every storage/ram/cpu combination on the MBA and MBP lines? I doubt it. It's still a fraction of what other makers produce.
That's swapping out parts obtained from third parties (hence BTO), different display sizes lead to difference in effective uses, and require very different internal chassis.