I wouldn't say it's almost double the size. The Air is about 10" and the Pro is about 13". That's far from almost double. If there was anything in between those two sizes there would be little reason to go with the Pro unless Apple had Pro-exclusive features.
1.3x is not really "almost" 2x.
We need to think in terms of area - to put it in perspective, would you consider a 500 sq. ft. apartment double the size of a 250 sq. ft. apartment?
Having said that, the ratio of the areas is (12.9/9.7)^2 = 177%, so still not quite 2x, but it is closer than 1.3x.