This split-second decision making process is quite interesting. Let's just say there are 3 seats open around the same area. None are designated disabled seats. All are aisle seats. What makes you pick one over the other? Something as simple as this actually takes a bit of quick thinking.
Here's what goes through my head in those 2 seconds before I must commit to a seat:
1) Does the seat have transferable stains? Does the floor have transferable stains?
2) Who is sitting next to the empty seat? Does he/she look pleasant to sit next to (or unpleasant)? Basically, if the person looks either disheveled, extremely overweight, heavy scented, rude/obnoxious, ill, unkept..etc., I try to stay away. The weight factor is usually not a huge deal unless coupled with another one of the listed characteristics.
For example, this morning, 2 aisle seats were open. I was quite happy since I felt pretty darn tired this morning and really wished to sit. One had a professionally dressed man sitting next to it, the other had an older woman who not only looked uncomfortably overweight but had just let out a scary, deep cough. Immediately, I chose to sit next to the man.
It's not prejudice for those of you who will immediately point to that. It's just a matter of personal comfort. I'm not saying people who fit those descriptions are not good people- they probably are- but I would just rather stand or not sit next to them on BART because of my own personal reasons such as allergies, being a clean freak, and having a bad day and need my personal space.