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.
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.)