I"m 5'11", now a portly 150lbs., at age 63.
I was competitive in all 3 engine sizes because I was lightweight (for 125's). For 500's, it didn't matter, body weight. For 250's, since there's lots of bar banging and leaning on, heavier than me was better.
I used to crack up watching the 80 pros graduate to 125's, when they were still 5' tall. Of course, 80Pro becomes 125 ExpertPro, so they only rode faster. Watched TysonVoland turn faster lap times at Marysville's sand track faster than OpenExpert. Some farm kids can really ride, until that big crash just before he signed the factory Kawasaki contract.