It could also be soil. Or, it could even have to do with precocity. For example, if your trees started bearing really young, that would keep them from growing, as from an early age energy would be expended on fruiting vs growing. I guess there's also a possibility that, like the nam doc's, there is more than one clone of the mc.
Will be interesting to see when more data comes in from other places.
You know.....root stock may provide some of the answer as to why different trees have different vigor levels. No control has really been done as far as testing the different root stocks. Not with Maha Chanok anyway. My original trees came grafted from Thailand on who knows what root stocks. I have since planted out two more trees on that I have no idea of the seed source other than it was from my yard and has about a 50-50 chance of being Thai Everbearing. Anyone know what kid of seedlings were used in the Excalibur trees?