I don't think it has so much to do with how many AIR molecules are going into the engine per se, more to do with how many FUEL molecules are. The power generated by an engine is based (all other things being equal) on the _mass_ of fuel burnt. The ratio of fuel to air should (if the carb's doing its job) be fixed, so as the air density increases (i.e. the temp decreases) more fuel is sucked into the bang box and more power is produced. So, as the engine is derated, the power limit is reached at a lower MP. As temp increases, fewer fuel molecules are sucked in, so you need a higher MP to generate the same amount of power.
Or something like that.