Having worked many years ago in institutional kitchens, the set up doesn't sound that odd to me. In a for-profit restaurant, it makes good sense to cost out every dish separately and keep track of inventory precisely, but for a kitchen in this kind of situation, where you have a set budget and aren't making a profit on the food, it's unnecessary accounting. Charging each meal as a single unit, regardless of what's eaten, is easier.
I like the ideas PPs have given, especially reminding her of her own advice "I've filled up on the delicious veggies as you suggested, MIL, so I really only have room for about half this steak, but thank you for offering to get me some more."
And, really, if she's financially well off (and what a blessing that is, eh?), the comments about not getting her money's worth are a harmless quirk, and I wouldn't pay too much attention to them. "Hey, you can get another helping of lamb chops. I paid for it, you should eat it." "Thanks, MIL, I'm good --- and I so enjoy just getting to spend the time with you." "Grumble grumble grumble." "Well, after all, MIL, I'm pretty much done growing (chuckle chuckle).... so I shouldn't really eat as much as a teenager, should I?"
But if she keeps making comments about it, what the heck. It's not worth getting into a sweat over, I think.