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The short form is that no one has access to the numbers really needed to prove that beyond the shadow of a doubt. There are two bits of data we do have to indicate relative sales, both of which are flawed.
Over time, you can follow the ICv2 and Amazon numbers for what they're worth, both are updated over time so this should be as valid a methodology in the future.
Neither WotC nor Paizo have been forthcoming with any sales data that would nail any of this down (some game publishers do release this info, but neither one of them does).
In the end, it shouldn't matter that much. It is nice for Pathfinder fans to see that their game isn't a fringe game but is rivaling the largest in the business, it provides reasonable expectation of continued viability.