Ah OP I'll share my story with you. I think you'll appreciate it.
DH is from Germany. We lived there for a while ourselves, and indeed lived with my ILs for a few months after he finished his studies. So I know what their eating habits used to be, almost 20 years ago, and they were relatively normal. If there's a hot meal, it's for lunch, and evening meal is abendbrot which essentially is bread machine bread (nice, seeded loaf), possibly some bread rolls, and a spread of various cheeses, meats, and a salad. It's lovely.
Fast forward to now, where they seem to have entered a competitive under eating competition where they are the only competitors. They don't get in any extra food when we visit now, despite us essentially being three more adults in the house (DS is a teen). Ok, fine, we're happy to go to the grocery store and get some cheeses and bread rolls to contribute. It's strange, because they're both retired from highly professional, lucrative careers (because I know someone will suggest they're destitute) and they raised three boys so they've some idea of how much food it takes to feed extra people.
Best example of a day, which happened before we'd reached the above conclusions so we hadn't prepared ourselves with provisions yet:
We'd had a late night flight in and the very next day, DH was off on an all-day stag gaming day with DBILs and friends. So we had no chance to go shopping, nor did I honestly think we would have to. DS and I spent the day alone with the ILs. Lunch was something small, or at least not especially carb or calorie laden (think vegetable stir fry). Totally fine, I'm not a big fan of massive meals. But it did mean that I was hungry by dinnertime.
Dinner was served. The aforementioned fresh loaf of bread was now FOUR slices of that bread, sliced in the kitchen and brought out in the basket as if it was enough for the four of us to have dinner. There was some cheese, and possibly a salami, and that was it. No salad, no other bread rolls, nothing. DS was starving so I ended up giving him half of my slice of bread.
Before you ask why I didn't ask for more bread: they'd portioned this out because this was the last of the bread, they were planning on baking a new one overnight and they assumed this would be sufficient for us.
I literally had half a slice of bread and a piece of cheese that evening. DH started texting me pictures of his fucking lovely meal out AND his fucking lovely ice cream he had afterwards and I was just so hangry it is hilarious to think about now. He got an earful when he got back later that night đŸ˜‚
We went out the next day first thing and bought in bread rolls and every other missing item in their house. They were grumpy but I couldn't give a shit by that point.
I've spoken to one of the BILs and his wife now and they've had the exact same experiences as well. It's gotten to the point where they won't stay there overnight, or if they do, they pack food and eat in their rooms so they won't have to deal with the judgement of the ILs.
So OP, I totally commiserate!