Yes, use the large chairs. Why should fats get special chairs just for them? Stuffing your face constantly is not a medical condition, it's a behavioral one, and they don't deserve for you to go out of your way so they're comfortable (which they will probably refer to as "comfy"), especially not if it means you have to sit in a less-comfortable chair. Oh no! Do they not have open MRIs where you are? Or are you claustrophobic even in those? {{hug}} That sucks, I'm sorry you have to deal with that.