Here's an airplane (an Airbus 320):

http://upload.wikimedia.org/wikipedi..._D-AIQT_01.jpg
It's about 37m long. It can seat about 200 passengers.

Here's a bigger airplane (the A380):

http://upload.wikimedia.org/wikipedi...tes_A6-EDC.jpg
It's twice as long (about 73m). It can seat four times as many people (850 passengers). It clearly is a bigger plane: it has two levels, windows for each level, and note the windows are the same size as on the A320. The hatches are all the same size, because they're built for the same thing (people). The cockpit windows are the same size. The cargo doors are the same size.

No one at Airbus decided to simply take the A320 design and scale it entirely up by a factor of 2, because that way, it would be a bigger, grander plane. The doors aren't twice as big on the A380. The windows aren't twice as big. There are design consequences to upscaling.

And before anyone tries: even bigger planes that have only one level (e.g. Boeing 767, etc....) have MORE WINDOWS that are the same size as the smaller one, hatches that are the same size, etc....