The reason so many dancers end up with that long, lean appearance has a lot to do with how ballet works your muscles. Unlike more targeted workouts like weight training or running, ballet works all along the muscle, not just here or there in bunchy masses. Having more muscle ups your metabolism, which also contributes to them staying small. Food-wise, Natalie did the equivalent of a crash diet, which any professional ballerina will tell you would NOT work in the real world. To survive their hours upon hours of training, dancers actually eat a ton. Extreme dieting or having an eating disorder can actually mean the end of your work as a dancer if your company finds out-- just like in modeling, a lot of companies have cracked down on the association between their art and eating disorders.
Anyway, if you really admire the dancer body, why not get into dance yourself? It's never too late I only just got back into dance a couple weeks ago after taking years off getting fat and I've already lost about 15 pounds. Granted, I'm dancing about 10 hours a week, restricting calories on non-dance days, and also doing yoga and cardio when I feel I didn't work enough, but it's really rewarding