who are downvoting and saying it's not right or fair, perhaps have minimal experience with aviation. This is a physics and business question, not a just a feelings and comfort question.

Weight is everything in aviation. It's perhaps the most important thing, other than general airworthiness the airframe of course.

An airframe has a very specific amount of weight — and distribution of that weight — it can handle. Added poundage has a dramatic weight in the performance of the aircraft. Ask anyone who's tried to take off in a General Aviation plane (i.e. a Cessna) just a couple hundred pounds over its gross weight. Compound that fact in warm weather. Climb rates are severely impacted, as is fuel burn, and other flight characteristics of the aircraft. Those factors can do result in a terrifying experience, enough that most pilots who didn't really think it was as big a deal as they were told, won't make the same mistake twice. It causes enough fatal crashes each year to warrant constant reminders from the FAA. Of course, with a large commercial jet, that margin for error and scale of significance is much higher, but it's still there, just with extra zeros on the end of the numbers.

When planning a trip, you don't just 'fill up the plane' with fuel, either. You weigh the precise amount (with a slight reserve margin) you're going to need to reach the destination — based on the weight of the plane itself at takeoff.

The FAA stipulates the estimation factor of what is considered an "adult" passenger, for the purposes of this calculation. Just like they have the baggage limit (usually 50 lbs) so they can use that weight limit x number of bags to get a maximum reasonable estimate for that weight. For passengers? It's 170 lbs in the Summer and 175 lbs in the winter. Yes, the weight is important enough that they actually stipulate a higher weight estimate in the winter due to coats and heavier clothing.

You think the airplane cares about and extra 5 lbs x 300 people, but doesn't care about an extra 300 lbs x 5 people?

In response to general embiggening of passengers, they're currently planning to raise those estimates to 190lbs and 195lbs, respectively, because consistently underestimating the payload on the airplane is leading to thinner reserve margins from a safety perspective, as well as thinner operating margins from the carriers' perspective.

This is, really and truly, part of the increase in airline tickets (among many other things). There are significant cost differences in flying a heavier plane.

So, defend the fat people all you want with your desire to be nice, but just know there an objective problem here. It's not really about feelings and comfort.