Perhaps I'm totally out of touch, but I'm not sure what you think you did wrong. Seems to me you handled things fine. They brought what they wanted, you offered a top up of home made fayre, but they wanted to get away. It's fine. All the more for you!

Why are you embarrassed? Didn't you do what they wanted you to do? You allowed them to bring the meal... small as it was. Some people are like that. They eat light. I have a sister and brother in law and cannot believe how little they eat. It was awkward that Christmas when they said, no one wants an evening meal do they? Erm, yes. :)