Neither my wife or I are Pooh Sized. We're actually quite small, 5'2" and under 130lbs (well, prior to my pregnancy!) I was wondering if that was part of the reason for the other passenger's confusion. I'm not sure though.
I did call SW customer service and was told to send them something in writing about the experience. They did apologize to me for the flight attendant's comment, but said that I needed to write to them in order to my complaint to be "brought to the appropriate person's attention".
So, I'll send them something off today. Nothing ventured, nothing gained.
btw: Next time we fly SW, we are still going to get an extra seat. It really made that much of a positive difference in our flying experience.