There's really not much more to know; it's a weight maintenance technique, since cellulose is indigestible. Force = mass*gravity and all that. The lighter they weigh, the more height they can achieve. Gymnasts have to worry about their weight for the same reason; that's why child gymnasts actually have an advantage over adult gymnasts, hence the Chinese gymnast boondoggle. But they can be stockier as they're always moving under their own power. Ballerinas are under even greater pressure to be light because their partners have to lift them.
I went googling but couldn't find any evidence of it actually happening; I did find this snopes thread in which it's discussed as a possible urban myth:
It seems to me unlikely, given that eating tissue paper is really difficult and in my opinion it would take far more willpower to force down enough tissue paper to induce satiety than it would to just stay hungry. It would be much easier to eat, say, a pound of celery, which would be calorifically indistinguishable from the paper.
Besides, real ballerinas must do a hell of a lot of exercise. I'm thinking they probably eat like athletes, not like teenagers with eating disorders.
