One more thing that should be considered is wingspan. A girl 5'9" could easily have a wingspan the same as a girl 6'1". It usually doesn't happen too often but it definitely happens. Maybe not the same but within an inch for sure.
All I have to say is My DD is number 17
and a girl 5'9" with wingspan of 6'1" girl would look kinda apish
Well he maybe right. Less distance or ball to travel, so ball is subject to less drag, less drag would give more speed. It may be so miniscule that you wouldnt notice but in theory he is probably right.
Is this an IR thing? I admittedly don't understand IR. I leave the instruction up to her coach so I haven't looked into it much. I hear her use terms such as arm whip and I haven't heard "Hello Elbow" since 10U so I suspect she's doing something simliar to IR, but I really don't know.
Whatever the method, my understanding is that a large portion of the pitching speed comes from the legs. Longer legs, more powerful stride, more speed? What am I missing?