There's a "sweet spot" as far as muscle mass goes for the best balance between agility and strength. Past that point, the negative impact from the extra weight added has more of an effect than the strength benefit from the additional muscle. Still, at least in the United States, relatively few female gymnasts are the stereotypical super-thin types. I weigh about 113 pounds now, but when I was competing I weighed around 120, and most of that extra mass was muscle.