@josekalel: I think the size of the terminal is kept in check for price reasons more than anything. This is a $100 set, and while bigger and pricier City sets exist, they tend to be few in number. What's more, the disadvantage to having a plane wide enough to seat two rows of passengers is that it inevitably takes up a big portion of the budget even for a larger set like this one. If the set were just the airport with no plane, then the terminal could probably be around twice this size at the very least… but then the set would not offer such a complete play experience.
This set is designed for children aged 6+, and one of the ways that it is suitable for them is the inclusion of separate instruction booklets and numbered bags for the different parts of the set (see my post about LEGO age range suitability here).  A younger modeller can build the plane or other item of their choice as soon as they open the box. It's not so much one big set as 5 different parts which make a whole scene. My 7 year old son chose to build a jet first of all...
Lego (Danish: [ˈleːɡo];[1][2] stylised as LEGO) is a line of plastic construction toys that are manufactured by The Lego Group, a privately held company based in Billund, Denmark. The company's flagship product, Lego, consists of colourful interlocking plastic bricks accompanying an array of gears, figurines called minifigures, and various other parts. Lego pieces can be assembled and connected in many ways to construct objects including vehicles, buildings, and working robots. Anything constructed can then be taken apart again, and the pieces used to make other objects.[3][4]

The remainder of the build is the control tower and terminal building. First there is a metal detector screener leading to the waiting/secure area of the airport, which seems taller than it needs to be for the minifigures. I think the same effect could have been made with half the height, except that would not allow for an impressive height for the overall tower. However it does seem to be similar to many modern airports with the high ceilings. This area also seems to be someplace to check luggage as evidenced by the rather nifty conveyor belt that comes in a full part. The moving parts of the set include a revolving door (a very tall one, for the first floor) that leads from the secure area of the airport out to the tarmac, and of course the conveyor belt.