Thai Airways just reduced their economy class checked baggage limit from 30 kilos to 20 kilos effective April 1st.
Bangkok Air has a 20 kilo limit but if you are a member of their frequent flyer program, free to join, you get an extra 5 kilos, down from 10 extra kilos last year.
Bangkok Air has a reputation for being overpriced on non-competitive routes such as Samui but I've gotten many good deals with them.
Air Asia has this reputation for being cheap but once you add in all the extras they aren't so cheap anymore and fly out of Don Muang.
I much prefer flying out of Swampy. Bangkok Air has free meals, free seat selection, and a free Lounge with light snacks, coffee, and wifi.