I dont see any real argument for saying that you'd need smaller fills for Xball than any other game. Admittedly, Xball is generally faster paced, but if you're playing a defensive point, it may be necessary to lock up and hold off the other team til the buzzer - dont want to run out of air then do you?
Also in Xball the turnarounds can be really tough, especially if you're short on pit crew, if - for whatever reason - you arent able to fill up between points it would surely be preferable to going out with half a 4500 fill than say 1000psi?