I have noticed that too, Air Asia let me fly even though my backpack was rather bulgy and somewhat over the limit... just try not to show it off too much around check in and you should be OK.
I've flown Kuala Lumpur Langkawi and Penang Kuala Lumpur and no problems on either one of those segments.... from Penang I even went stand-by as I got to the airport early, and was very pleasantly surprised with Air Asia.