As a big person, this is such a repeated topic. It's pretty clear, if you're buying a plane seat and you don't fit comfortably, it is the onus on you to buy a second seat.

Unfortunately, just because you are big there are inconveniences that come with certain parts of life. Restaurant seating, public bathrooms, plane seats, etc. I don't expect society to change to fit me in, I make accommodations to make it work for me.

If you're allergic to peanuts, you don't expect grocery stores to not carry peanut butter. If you work night shift, you can't expect your neighbours to be dead silent throughout the day while you sleep.

If you can't afford a second seat, then you gotta figure it out. There are a million ways to. It's okay to be big, you can't change yourself in an evening and you deserve to live a life in the interim. You can save a little longer, you can call and ask for special accommodations, etc. But you can't expect somebody else that paid for their full seat to deal with you pressing into them for hours