I was skeptical but just had to try it.
I started with a Creamy Supreme Classic White frosting from Pillsbury.
Here it is in the bowl before I whipped it:
Not sure it quite DOUBLED, but it did increase in volume. I did notice at the grocery store that the same size can of frosting for the whipped version weighed less than the non-whipped version, though not half as much. So that tells me that this is a legit tip, in theory. My only thought is that it isn't doubled, but it is increased (in my experiment anyway). Obviously if you're adding air to it, it's going to fill it out. Good tip!