@takeadaytomyself2025you’re not fat, you don’t look fat, and you are slimmer than the “average” UK woman by a decent margin.
Whoever it was that said that to you is a twat, for trying to insult you by calling you names, and for being plain wrong anyway. You surely don’t think they are right? And even if you were fat, so what? It’s something that’s said to women to try to shame us and to make us feel inadequate for just existing in the world.