Why is the sky blue?
Why do stars twinkle?
I'll try. It's something I've observed.
Shorter players tend not to hit 140 mph serves. Meaning, their swingspeed is less fast than taller guys. With a fast swing speed, you want to be able to use that fast swing speed to get the ball IN, every time, on second serves.
Back to shorter players. Slower swing speed, can we agree? Function of less leverage and that sorta stuff...no disrespect for OliverRochus.
So a sidespin second serve can arc IN with assist from gravity, lack of ball speed, air resistance, BUT, a lower strikepoint leading to a smaller target area...which is the counter, but maybe here offset by the slower swing speed and slower ball speed.... MAYBE. We are speculating here.
And, almost as important, since shorter players generally tend to stay back and play a more defensive type of tennis, they don't NEED a really fast moving ball. In fact, a slower moving serve allows them to get in better defensive position to defend against a hard return of their serve!
Offense and defense require diferent criteria to succeed. Speed, surprise, quickness is advantageous to offense. Control is vital in defense. Control is helped when your serve allows you to get in defensive position before the return bounces into a corner of your court from the return.
Possibly, is all I'm saying....