Interesting.
There was piece in the rag a few weeks back where the AMA were flapping their gums that BMI was no longer considered a good indicator of a persons health.
i.e they worked out you could simply be a vertically challenged walking ball of muscle that skews the BMI result.
Using BMI as a guide to a persons ability to enter/exit an aeroplane is a crock. I know (sorry) elder guys that easily fit into the BMI index, but due to age have mobility issues.
Personally i think having a larger BMI helps aid your pax exiting, becuase you are large enough to 'aid them on their way out the door'