Average seat pitch is 31 inches these days, but the room you would have from buttocks to your knee would be smaller than that. 22 inches isn't uncommon: http://si.wsj.net/public/resources/images/PJ-BY155A_MIDSE_16...
"Buttock to Knee length" seems to range from about 21 to 25 inches: http://ergotmc.gtri.gatech.edu/dgt/images/hbfg0713.gif
It's not hard to imagine someone with 24 inch+ b-to-k having trouble fitting them into a 22 inch space.
My son is 6'3", and has real issues with this.
