The first few bags build the small truck with carts that you see on airport runways. The truck has a "follow me" sign that I confess I have not seen at any airport. There is also a fueling tank and a rolling staircase that sits next to the plane to allow for boarding. Both the tank and the staircase are additional carts for the truck, though I am guessing not both at the same time.
@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.
Bag 4 finishes the fuselage and all that is left are the engines and tail section. You'll notice that this aircraft has a bathroom on board as well as some storage for a non-existent flight attendant. I think this is the first time that LEGO has put some additional details into aircraft besides just seats. Also there is only one pilot for a 2 seat cockpit in the set.
Since around 2000, the Lego Group has been promoting "Lego Serious Play", a form of business consultancy fostering creative thinking, in which team members build metaphors of their organizational identities and experiences using Lego bricks. Participants work through imaginary scenarios using visual three-dimensional Lego constructions, imaginatively exploring possibilities in a serious form of play.