Certainly, from half a world away, I could have easily missed the mark on this one, but I hope you didn't miss the point. I do not know the exact genera of the bird you had, but I still bet it was some kind of flycatcher. There are nearly 40 different ones recorded as being resident in Thailand. The previous picture was just a handy example of one species, of which there are over a half dozen subspecies. Within the species, they are very different in size when adult.
To illustrate my point, here's another flycatcher resident in Thailand.
I'll stick to my flycatcher theory, but will courteously back down if proven wrong.