In this instance, you chose solely her negative physical attribute, fat, to identify her rather than calling her the lead actress, the blonde girl, or use the character's name. You may want to ask yourself why you did that and if you'd like to be identified in the same way. Also, it may bother other people who you're talking to if they hear you identifying people in that way, like you might similarly reduce them to a single negative trait.

That said, people do a pretty bad job of explaining this and immediately jump to level 5 "I'm extremely offended."

In the future, when you're referring to people or identifying them it's worth thinking about how you do so just so you don't touch on something that may be a struggle for them or really hurt them. Weight and other things can fall into that category. That one member who jumped all over you may have had weight struggles. Think of it as conscientiousness, not political correctness. This is a courtesy to the people you're directly interacting with.