I wouldnt say tighter, I can just say I've never been with a loose Asian girl.
From what I understand, it's based on how nervous they are, since they tend to get tense. That's why as you start having sex with that girl more often it is never as tight as the first time, because now they are more comfortable and the muscles relax.
Not trying to get all doctor about it, but that's what I've heard and experienced.
This one asian girl back in high school, I couldnt even stick a pinky (barely got the tip in) in there and she had to wear pads because she couldn't wear tampons. Sad to say, I never took her virginity. Damnit I tried though...