At the Grocery Store.....?

I think you overreacted. All that was necessary was a simple "Excuse me, please", and she would have apologized and moved. Happens all the time. People do rude things often without realizing it. If you are polite to them, they will usually react accordingly. Think of it this way: How would you have reacted if she had said, "Sorry I am thinner than you, but I still need to get my receipt"? Just because she's thin didn't give you the o.k. to compare bodies. Some people are sensitive about such things.