Probably more on BMI. If you're 300 lbs at 6'4, might just fine. But I had a companion that was 265... And he had to lose 50 before his mission.... 5'8 315!!!
We had a missionary that was about 6'3 400 lbs. Mama would probably be just fine. And when you look at him, he's not a big blob of goo. He holds all 370 fairly well.