Great answers!!
I asked about this to our L9 coach while chatting sometime back. She was more then happy to share her thoughts.
The first thing she stressed was to be wary of ANY coach that tells you that you need to loose or gain weight -- just because the coach says so. That's dangerous territory and has led to countless eating disorders among athletes. Unless the coach is certified as a sports nutritionist (which precious few are), they really aren't qualified to make those determinations and could be doing more harm then good.
She also mentioned (and I hope I remember this correctly) that mathematically, there is no difference between a person that is 75 lbs or one that is 150 lbs IF both athletes have the same differential in strength and endurance. Meaning: A 75 lb person may be required to move and maneuver 100 lbs to perform with endurance. Whereas a 150lb person would have to be able to move and maneuver 200 lbs. If both can -- then they are equals in strength and endurance. The lighter athlete won't have any advantages due to weight.
As mentioned, muscle weighs more then bodyfat. This means that building muscle and strength can also mean gains in weight. These gains can be misunderstood as "excessive" weight - And it's not. It very well could be the muscle that you worked so hard to build and strengthen. There are ways to measure bodyfat (google it), and from what I've read, bodyweight scales should be avoided as a measuring tool. They simply can't determine what's fat, and what's muscle. Nor can any simple mathematic formula based on height & weight.
For example: Some may claim Shawn Johnson is fat. Compare her to Nastia...and yes, she may look a little heavy. What they don't get is that ‘babyfat’ looking girl is a very muscular. Olympic weight lifters and pro football players weigh in at some 350+ lbs, and are only 5'8" - 6' 4". Are they grossly overweight? If you used a weight scale or a height/weight forumla...they are clearly big fat heffers! See the difference?
If I recall, being in the range of 12-15% total body fat is considered "athletically healthy", with 15-25% being the goal for us "normal folks". (Our national average is...much higher in reality). If you venture less the these percentages, the risk is deterioration of muscle. Muscle that took a very long time to build up in order to perform. Why would any athlete want that? Just something to rememeber when you hear of athletes that whittled themselves down to 2% total bodyfat. Bottomline, if they did, all they are really doing is hurting their own chances of success.
Listen to the coaches when they say: EAT properly, get your rest, etc. (avoid soda...even diet soda. it's all evil! Drink water, and natural juices.) I also recommend reading up on proper nutrition for athletes. Blairbob is absolutely correct. Young athletes don't know how to eat properly. And parents (shame on us!) don't learn how to feed them!