Not just AA. The UA 737s also have very narrow underseat space on the aisles in both F and economy. My backpack will barely fit if I remove my chargers (laptop, other devices) from the side pockets). Forget briefcases, as you can't really reduce the width. It really is ridiculous, as I've seen many people have to resort to using the overhead bins for their bags which they would have otherwise put under the seat.