Why is everyone mixing up sensor size with megapixel size? Both arguments are correct. If you establish a size for the pixels, then 36 is double 18, BUT this is not equatable with sensor size, even if the receptors(pixels) are jammed in there.
We need the optical physics guy here. Larger sensors have optical physics properties, regardless of megapixel count, that smaller sensors don't (due to light gathering area), but don't confuse that and think that more megapixels is the equivalent of a larger sensor.
It might have the megapixel count of medium format, but it is definitely not a medium format sensor. Hasselblad and Mamiya are not quaking in their boots over this. But it is cool to have the MPs, though I'm not sure that the file size is worth it unless you're trying to print billboard size photos!