I have travelled a lot on AirAsia Thai and had to pay excess baggage for 1 kg out of BKK. I do not trust Air Asia or any low cost airline on these things. As some other OPs has said it is dependent on the individual agent and you never know what they will do. My experience has been they are more strict on the weight and I also have seen many outsize bags on board especially on the domestic flights on Air Asia Thai.