The CAAP data and the recommendations come from a study done by one of CASA's airworthiness engineers (JK) quite some years ago.
The study was based on Australian medical anthropometric data. The basic concern with aircraft size is the effect of population standard deviations. On the smaller aircraft (as I recall, the cutoff adopted was 8 seats ?) the variability makes the use of standard weights a bit unworkable.
John's study probably is still available from CASA .. my copy is tucked away safely in the files.
If you are in the Canberra area, John is semi-retired and probably would be quite happy to take you through the background for the price of several coffees ..