I think as an overweight person, you're pretty much on a hiding to nothing. You want to sit, as much as the next person, and if you're overweight, standing for long periods can be particularly difficult. So what do you do? Ignore an empty seat? Because you KNOW the person next to you is going to think you're imposing on their space? What choices do you have? Stand for the rest of your life so someone else doesn't feel uncomfortable?
I used to be overweight and the number of people who would grumble because I sat to them on a plane or a bus was disgraceful. I paid for a seat, same as anyone else. A bag is something someone can help, weight kinda isn't. I don't think it's rude for me to want to sit down, as everyone else seems to have the right to.
Like I said earlier, if someone had said to me to move because I was hurting them, I would have. Whether I was overweight or not, such as the skinny chick who practically sat on me on the tram. That's why I think this woman's rudeness and weight were un-related.