Every major airport is in the same boat.
KDFW is completely full (save for 0100 to 0400), I'm lucky to control 55% of the slots, but I've got another 150 planes on order.
Not every new plane is added to the existing fleet. With as long as games are, to maintain a good image status every plane you own must be replaced at least once throughout the game. So a carrier with 200 planes now is going to have to look at replacing every one of those in the '80s. That means ordering replacements now.
With as long as you wait for planes, you have to buy them with the assumption that you'll force competition out by the time they arrive and expand your slots. The gamble here is that you better have an alternate game plan if this doesn't happen. This is the danger of leasing planes. If you bought outright and odn't need them, you can just sell them. But get yourself stuck in a lease contract with planes you can't use and you found a recipe for quick bankruptcy.
With newer, larger planes you can put MANY more passengers through the same number of slots. You might have 4 flights a day with a single A320 to Rome, for example, but you can actually replace that plane with THREE additional, larger planes on routes with much higher margins if you know what you're doing. You buy a 747SP to fly that route instead, it then only has to fly once a day and can afford another flight somewhere else too. You're now making twice as much money, and still have 2 slots that you freed up by cutting the A320 flights to Rome, you can put 2 747s on long-range flights to use those 2 slots you have left.