How thin is “just right” for someone who says they prefer slim women?

I’m currently 161 cm tall and weigh 47–48 kg. I gained 10 kg over the course of a year. All the men I date say they prefer slim women, but whether I was in the 30-kg range or at my current weight, they’ve all said my figure is “just right.” In fact, no one has said anything even after I gained 10 kg. When I say I’ve gained weight, they tell me, “You really don’t look like it.” They don’t seem to be saying it just to flatter me—they genuinely seem to think so. Does gaining or losing 10 kg not show that much in your figure? I don’t really understand what “just right” means to someone who says they prefer slim women.