What's critical is the hip measurement (unless the waist measurement is bigger) and the size of the sett. Someone with 46" hips might need only 8 yards if the tartan has a 7" sett, whereas someone with 42" hips might need 9 yards if the tartan has a 10-11" sett. And there are tartans out there with huge setts. The biggest I've seen is a 16" sett (and that was, in fact, kilting tartan!!!).

And, I agree. There is no way you could use 9 yards in a great kilt! You'd be completely muffled.