To NYChic and everyone else concerned about the space they paid for being encroached upon, as a woman who is on the sizing borderline between missy and women's sizes, I too, do not like anyone else invading my personal space. Perhaps we would all be best served if we complained to the airlines about the ridiculously "sardine can" seating that they offer everyone save first class. I have never heard anyone, including height-weight proportionate people, say that they have adequate room to be comfortable. Let's work for meaningful change, instead of bickering amongst ourselves, accomplishing nothing.

You are not in the least inconsiderate. You're perfectly correct. Being of large size does not entitle one to special privilege, not does it entitle one to make others uncomfortable or to take from them their property (e.g. a legally purchased airline seat). If someone chooses to be large, that is their business, and they should not be discriminated against in areas where their size does not hurt others. But they are responsible for their choice, and for paying for two airline seats if they cannot fit into one.