They don't. Doors are designed to meet certification specification and nothing more. Additionally, you have main cabin doors for egress for anyone of size.
You won't get anyone too big sat next to them on most airlines as extension seatbelts are not permitted at exit rows. That's not to say people from rows other an an exit row won't try to use the exit but let's face it, there are many people for whom over wing exits are not suitable for, both in terms of getting through the exit and then getting down to the ground.
Let me guess what has happened here. You're slim, you've been pulled for tour bag being too heavy or too large and you think it in unfair that people weigh more than you don't have to pay extra?