When they show the image that is half prepared burger and half store bought burger (3:20), you can see that they're almost exactly the same height. We perceive the store bought one to be smaller because it has cheese covering up most of the patty and melted on (whereas the prepared one has the cheese resting on top), it's darker/less vibrant, and the bun is hugging the patty tightly, which makes the patty seem smaller when compared to the space provided by all of the toppings in the specially prepared burger.