If you render the pixels they will have a physical size. MP is a scalable measure of area. Whatever area you assign to 1 pixel, 2 pixels will be twice that area. Whatever area you use to render 16MP, 32MP will be twice that size.
well you mix things up here... MP have no unit of area assigned to them per definition.
only if you give an additional information (1 pixel x 1 mm^2) you give them a dimension.
you can print 100 MP as 10x10cm print and you can print 10 MP as 10x10cm print.
so nowhere in a megapixel number alone.... you have information about size.
and that´s what im saying.