I am fat enough to need an extension belt on some aircraft, and in my experience it virtually never occurs to BA crew pro-actively to offer one: I always have to ask. I would be delighted if my need was identified without me having to attract attention and ask every time.

Had the crewmember called you fat, you would be justified in complaining. But asking you if you want an extension belt is not 'effectively calling [you] fat' except in your own mind.

I have learned to accept that anyone I encounter who isn't blind may consider me fat (or worse), which in my case is fair enough because I absolutely am! Perhaps some people who encounter you may think you are fat, too. Who knows? What I do know is that you have the same level of control over what people you meet think as I have, i.e. none. I really don't think what was said to you would have affected the views of other passengers, and whilst I understand you may be upset, I really don't think you have much basis for a formal complaint.