I was reading a webpage a small sampling of what thin people tend to eat. Many of them would have soup for lunch. I don't find soup at all satisfying. I wonder why it is that I don't.
There is a reason that "soup and a sandwich" is a cliché. But your thin person isn't like any of the thin people I have known well. The ones who lived at my house always ate way more than everyone else. So for me, soup would be plenty for lunch, but for a the thin folks they would need a sandwich or two, and some fruit or pudding, and maybe an after lunch snack of a muffin, to go with the soup.