I can see how some people would imagine that as 'fat'.
We've been raised to look for Barbie, Twiggy, Kate Moss....
Look at this image of a
runway model & a healthy 'normal' woman.
Your photograph looks like the woman on the right, body fat percentage-wise. There are people who expect every woman to look like the woman on the left.
We've had people complaining about one of Australia's
Olympic athletes, claiming that
she is fat!! She qualified fifth fastest in the world in her trials. If someone capable of that is unfit, I'm a banana. And Irv's that grilled cheese sandwich.
Anyway.
It wouldn't matter if you were burn-scarred, disfigured, overweight, anorexic: there is no right for anyone to say those things to you.
Part of it is unrealistic expectations.
Part of it is that you could have a better photograph.
Most of it is that they're jerks. And by 'most of it', well, I guess I mean all of it.
(BTW: in the Gimp, to de-yellow that pic, experiment with the tools in the colours menu. Particularly 'color balance' and 'hue-saturation'. You can also use 'levels', 'curves' and 'threshold', which take a bit more experimentation but can give you a more precise result.)