This policy isn't aimed at those people who comprise 60% of the population. It is aimed at those who take up more than one seat and generally weigh more than twice as much as the standard 160 lb average person.
Quit justifying obesity with soft PC terms like "weigh just a bit more" and "fluffy". Just a bit more does not mean one seat more.
We all have horror stories about being seated next to a FAT person. Good grief, I am married to one.
If this is a problem with the flying public, then a wise airline will offer an alternative for a profit.
Heck, I would pay double for an airline that allowed smoking!