If you are not willing to invest in the first (or even business) class seat that is available, wider, and often not so crammed next to the adjacent seat, then your only alternative is to lose weight or drive to your destination. I work out 5 days a week, count calories, and I am not ashamed of having a healthy BMI. As soon as a passenger of size sits down next to me, it is automatically my responsibility to have my comfort impaired the entire flight.
If I say anything, I’m a “size-ist” or a misogynist or an agent of image-conscious patriarchy. There’s no personal hatred towards larger people, but it isn’t fair that your unhealthy life choices should be accommodated by others.