Airlines have two options for making the seats wider:
1) Make the aisles narrower. They're narrow enough already. Not a realistic option for reasons of safety and comfort.
2) Take a seat out. Five abreast instead of six on 737s and 320s means tickets will be considerably more expensive for everyone, including me at 1.75m/66.6 kg. Not fair. And congested airports will need to handle even more flights. Definitely not a good idea.
Cabin staff can make people check excess carry-on luggage. Tall people can get exit row seats. But for fat people who don't fit in their seats the only options are flying business class, buying two economy seats, or lose weight.
If you bring more than 20 kg of luggage on a typical european flight in economy, excess weight is charged by the kg. Why not apply the same to passengers? Americans can keep their own traditions and charge by the inch instead.