Thai has people coming in from around the world in 'waves' of arrivals and departures, so the first and last few flights of the day to and from Thai domestic destinations like Chiang Mai and Phuket are packed full of transfer passengers, hence the need for 747, 777 and A330 on these flights.

Also - the widebody fleet can spend a whole day in Bangkok between flights so they have time to operate a few domestic legs.

You won't know the configuration of the aircraft until 24 hours before, and even then it is subject to change at the last moment, and regularly does. You will be quite comfortable in business class for the one hour sector on any of the 777, 747 and A330 aircraft.

Other airlines operate widebodies domestically on short flights where the capacity is needed. In Japan, the Philipines and Australia for example.

cheers