I agree that there are some differences. The A321 is the same size as the 757-200, slightly larger then the 737-900. However, the 757-200 is a much, much more powerful aircraft that can do a wide variety of flights (Trans Atlantic, short runway, etc.) that the A321 is incapable of. Also, the A318 is roughly the same weight and size as the 737-600 and not the 717.