Any would -be ballerina suffers from the fear factor in developing big butts or big thighs instead of the long lean look. Especially if that shape runs in the family. Especially when human genetics are discussed as though no one can escape their fate.
Your genetic shape is modified by what you eat, and how you exercise.
Posture is everything for growing to be long and lean. The spine and pelvis must be in a natural position. A postural plumb line must be straight from the top of the head, down through your body's natural curves, to your ankle bones.
If a mirror at the ballet studio, or at home, reflects a vertical line of some kind, like a door frame, stand at the mirror so that you can place your body sideways, in front of the line. Notice how your spine and legs line up along the line. If your weight is leaning back, or too far forward from your ankles, you will be able to see that you are not standing along the line.
Your spine has three natural curves. The most noticeable curve is at the waist area of the lumbar spine. A common misconception is that this curve should be pulled long, by tucking the pelvis under, and that this stance protects the low back. Exactly the opposite is true. Years of tucking under will lead to low back pain and perhaps even herniated discs. Correct posture will protect the discs. The discs, in turn, cushion the bones and allow maximum comfortable movement.
This tucking under of the pelvis will develop big thighs, or quads, and big butts, or gluteal muscles. The body weight presses down on the thighs, not allowing any elongation of the muscles. The deep rotator muscles are not in a prime position to hold turnout, and the gluts have to be held clenched in a bulked up fashion, to keep the pelvis stable.
The long lean look of a ballerina is from working pulled up in the deep lower ab muscles, and with a feeling of pressing down through the center of her leg bones, as if pushing the floor away. The thigh muscles pull up but do not clench hard. You need a certain amount of tension for the stability of each ballet position and movement, but not more than that. Extra tension will create less fluidity of your ballet movement.
Pilates is an excellent cross training for dancers because all exercises are done emphasizing length.
If you understand the correct posture that a ballerina works in, you can learn how to get the long and lean look, and avoid the big butts and big thighs that develop from working in an unnatural way.