At least he was kind enough to offer his seat. And you saved him embarrassment by taking it instead of pointing out the error. I think the situation went as smoothly as it could.
Maybe you can use this as your wake up call to finally get on a path to losing weight if it really bothered you.
Mine was when I saw myself in a picture about 2 years ago...I was so big, my face looked like it was swollen from a bee sting or something.
I was disgusted with myself, so I started doing something about it. 35 pounds lost so far. :)