No, I think the practice is discriminatory. Not everyone who is larger than whatever is considered the "norm" (and with the airlines, who decides what is normal height and weight?) is able to change his or her size. Height, of course, is not changeable, but some people gain weight due to medical problems, medication, inability to exercise because of disabilities and limited mobility, etc. Many things cause weight gain through no fault of the individual and are difficult, if not impossible, to change.
I realize airlines have very high expenses, of which jet fuel is major, and the quest for market share of airline tickets sold at the highest rates possible drives their business. I've thought for years that if airlines made their coach/economy seats larger and gave travelers more leg room they would build a base of customers through loyalty that would increase their fares sold--not that anyone from the airline industry asked me!
Discrimination against people who don't fit into a particular short, skinny "mold" decided on by an airline is not good customer service, and we all know that people who aren't forced by circumstances to put up with it...won't. It seems like a good way to lose customers. By the way, which airline does it? (I'm just curious, as I rarely make use of air travel since I retired.)