That would never happen.
Presuming that your using lettice as a base its impossible. What happens to lettice when its in hot water, Its wilts and shrinks, just like it does in ur belly.
Well how big are these salads your eating, first off? And how much other crap comes on them? Your stomache could expand if your eating way TOO much all the time. But if its just a small, regular salad, don't even worry.