Habs Fan,
It is not the size of the Immigration/Baggage hall that is the problem. It is the fact that there is only one corridor leading to Immigration or to the Aircraft. As you cannot have passsengers mixing i.e. incimming and outbound passengers. The only alternative is to close the corridor to the incoming passengers until the outbound passengers have finished boarding their aircraft. Since we now live in the age where Airlines and the general public want value for money the aircraft can only be scheduled for a 45 minute if you are lucky or 20 min turnround if Low Cost. This means that priority is then given to boarding passengers rather than allowing disembarking passengers to use that one corridor.
Its all to do with money I am afraid to say. You getbwhat you pay for in the end.