The OS is completely different. Maybe if you want to argue an ipad 3 or 4 versus a MINI, then that is more understandable.
It seems fair game to me. Imagine I am in the market for a tablet. I have a choice between the Nexus 7 and the iPad Mini. People compare them in real life, so why can't we do it here?
Of course, the OS matters. But, so does the ecosystem, your use case, your budget, your wants, your needs, and all sorts of other things come into play. I have known people who chose the Nexus 7, for example, because it is smaller. Or, because it has better screen resolution. Or, because it has better apps.
Anyhow, this thread isn't really about the Nexus 7 vs. the Mini. It is about (I guess) the imaginary number of 2 Minis sold for every 1 iPad. So, you are right to bring us back to the large iPod vs. the medium one (as I like to think about it), and to leave out the Nexus. For me, the iPad fits my use case perfectly, and the Mini is a very poor fit. But, for the little girl at the airport I saw last week playing games on the Mini, I'd say it was the perfect size. To each his or her own.
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[EDIT:] By the way, we now have some more numbers. It looks like iPad Mini sales are not quite as brisk as the OP suggested.
http://www.dailyfinance.com/2012/12/29/will-the-ipad-blowout-this-holiday-season/
These are just guesstimates, of course, but I think the thing to note is that supplies were supposedly not meeting demand, but will in the next quarter, especially in China. And, interestingly, it is not a zero-sum game, but the tablet market continues to grow, so there is room for Apple to gobble up even more customers. I suspect they will need a full line of devices (small, medium, and large iPods) to cover the spectrum of users. I just wish they would beef up the small and medium specs...