I actually know the history of the iPad mini quite well. I'm something of a fan of small tablets.
The Mini 2 was the one and only time the Mini was at complete parity with the larger model of the same generation. The Mini 4 is actually the next closest example, as the only difference between it and the then most current iPad, the Air 2, was an A8 rather than A8X. If memory serves, in single core performance the A8 isn't even that far behind the A8X.
I don't know why or how they were able to have the Mini 2 match the Air (and actually, if you want to split hairs the A7 in the Mini 2 was slightly underclocked compared to the Air, which supports my statement about smaller devices and weaker processors) but the one example of parity is the exception, not the rule. From the very start the iPad Mini was introduced with the internals of an iPad 2, at the same time when the iPad 4 launched. The Mini 3 was a full generation behind as it had no internal upgrades (now THAT was a rip-off!). A general rule of thumb in technology is that making the same thing smaller is more difficult and costly. This is (in theory) why iPhones cost so much more than iPads. Yet every Mini has been less expensive than the corresponding 9.7" model. You can't have your cake and eat it too... Except with the Mini 2 which really was an anomaly.
So given that the Minis always cost less than larger iPads and have to cram more or less the same technology into a smaller frame, no, it's not really fair to compare them. I compare Minis to the previous Minis and to other tablets in the same weight class, so to speak.
But if an iPad, is an iPad, is an iPad in your eyes, how do you feel about the fact that the newer 9.7" Pro comes with a weaker processor and half the RAM as the 12" Pro? It's as far behind its big brother at the Mini is behind it, if not more so.
"Study the history of the iPad Mini..." LOL.