Hmm, Sayre beat me to starting the thread. OK, here goes.....
Based on my own experiences failing at weight loss for 30 years, finally succeeding four years ago, reading accounts of people losing weight in books, blogs, alt.support.diet, and Bike Forums, discussions with fat people, and conversations with people who have lost a lot of weight, my understanding of "thinking like a fat person" is that it breaks down to three parts:
1. denial;
2. learned helplessness, and;
3. the habits that reinforce the two points above.
No two fat people are alike, so the proportion of these items varies from person to person. Also, there may be other factors - health problems (Tom Stormcrowe could comment about the disorder that brought him to 562 pounds), psychological problems, abuse (I'll raise my hand here), addiction, etc, that complicate matters.
Denial is an obvious item, so I'll skip straight to learned helplessness. The severely obese, like myself, develop the mindset that no matter what they do, they can't succeed, or if they do, it's not enough of a success, or it was luck, or it's not important. Or they manufacture ways to avoid situations in which they may succeed, or set themselves up to fail. Or settle for less, because that's the best they expect.
An example from my past: when I was at Antietam last year, I made all sorts of excuses to avoid riding the battlefield. It was too tough, I was fat and sluggish, it was late in the day...... then I told myself I should do what I wanted to do, not what my past told me I should do. I had a great time once I stopped thinking like a fat man.
Another - last Saturday, after riding with Sayre, I stopped for snacks at a gas station. The woman behind the counter flirted with me. She wasn't my type, so I was polite but cool in response. On the way back to the car I was downplaying what happened - "she must be very desperate to flirt with a fat guy like me." For a second I saw myself as 400 pounds again. I didn't even consider the idea that I might be a handsome - OK, good-looking - guy. Fat thinking strikes again.
Sayre's post above details some specific behaviors and habits, the "old tapes" of learned helplessness. He doesn't sit in booths because he still thinks he won't fit, or that he doesn't deserve to sit in a booth.