I'm pretty sure it's just a genetic difference, all the Thais i've seen (aside from top-tops like petboonchu, buakaw, pakorn etc who sometimes work with personal conditioning coaches) train the same conditioning wise, aside from different styles of fighting, padwork etc.
Some Thais have that Tyson, Manhoef, etc. physique (NOT the exact physique, i've never seen a Thai that big, i'm talking about the "type") where they can hold onto alot of muscle despite lots of cardio. But they are very far and few between. Most are skinny/lean, or just thin.
I'm pretty sure the above guys i mentioned (Tyson, Manoef) juiced at one point, but that doesn't change their natural bone structure and propensity towards being heavily muscled.
Just like in any group of people there are variances in physical traits.