Could it be that 10 AEF is on a coastal airfield?
All cadets flying from Woodvale need to wear the LSJ, which means (IIRC) that the minimum size of parachute is medium. That puts all of the smaller sized cadets out of the picture.
10 AEF can not give diffinative minimum cadet sizes (as it's down to the survival equipment fitter the day), and if you do nominate a cadet who is judged too small, the AEF will email your wing about the 'wasted' slot. I would hazard a guess that most squadron commanders err on the side of caution, and turn down slots unless they can fill them with larger cadets.