There's more than zero data and coaches are able to see pitches during tryouts or other evaluations. However the eye test still exists, coaches are biased and imperfect, kept game changer stats are biased and imperfect.
Small players just need to be way better, like so much better that it's indisputable, that no matter how much potential a bigger girl has, she cannot surpass her, because like it or not smaller players have a lower ceiling. Coaches always look at potential, and think that the taller girl will give their team a higher ceiling by the end of the season, even though that may not be true.
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I think this is especially true with tryouts and practice. Watching some kids throw to a catcher without a batter or umpire they just look so fluid and natural can't help but think they are better.
When you interject a hitter all of a sudden the kid with little funky motion or change of speed or more spin does great. That same kid who looked so natural in tryouts gets hit because the ball is easier to time or maybe flatter.
The good news is often some of the smallest players end up being truly great because they always had to outwork bigger kids that were more natural. Have to believe Altuve was passed over more than a few times growing up because of his size, but that likely made him reach his true potential.