Those ketchup cups you use at McDonald's are expandable. Maybe most people know this, but I only found this out recently. I do know that I've yet to see someone actually expand the cup before filling it with ketchup. They have the potential of holding so much more than the assumed 15 fries worth. (That's my standard use of ketchup, not that of the average fry consummer.)
My son showed me this too. Only there not meant to be expanded. If you do "expand" them they become like a flat saucer. Could you do this so that you could hold more ketchup? Sure. Is that what they were designed to do? Nope.