If the purpose of weight guidelines is to enable the FAs to perform their job efficiently and move down the aisles speedily (as seems to be the case as shown by the Qantas guidelines on page 2), then why would BMI be relevant? BMI is a height/body mass guide - you can have high BMI but still be small and thus still fit through the aisle comfortably.
The test surely is simply whether you can do your job properly and move down the aisles with ease. This in itself (see Jessica Starmer/BALPA thread) might be discriminatory, since the anatomy of women generally lends itself to wider hips than guys.