This is a weird situation and I want to hear what you would have done in a similar scenario. The train was crowded with people standing. There was an extremely overweight woman who took about 1.25 seats, with the edge of her thighs hanging over to the empty seat next to her. People sat down for a second, and then left in favor of standing instead. This happened through 3 stops. As the train got more crowded, the overweight woman still sat alone, with her head down, looking a bit awkward with each person next to her leaving almost immediately.
I felt bad for her. It was clear that people didn't like sitting next to her. I was already seated but decided to get up, stand for awhile as if I'm leaving, and then, took the seat next to her. As I sat down, I said hi, and she whipered hi back. I was plastered next to her, but I wasn't uncomfortable.
I don't know if she cared that people didnt want to sit next to her but as more eyes stared towards her direction, I just felt like I needed to help make it less awkward.
I can be imagining all this of course! But oh well...it made sense at the time!