Elite female gymnasts are getting smaller, because smaller gymnasts have a better power-to-weight ratio and a lower moment of inertia — but there’s a bit more to it:
In the past, the judges gave taller, more elegantly moving young women advantages because they looked better. But that gave an advantage to Eastern European girls raised in the traditions behind the Bolshoi ballet. The Americans have lobbied to make scoring more objective, which gives the advantages to Mary Lou Retton-style muscular human cannonball body types like Biles’.
Same thing holds for figure skaters, who are also getting shorter as the gymnastic requirements are getting more extreme. In the sixties and seventies, the dominant Russian women were always compared to ballerinas.
The exception is males in pair skating; the men have gotten bigger to lift women higher and throw them further. Singles male champions are mostly short; Scott Hamilton is maybe 5′ 3″.