Urgh. So many answers that the real bun is shrinking due to the steam - that would make the real patty look thicker than the ad patty, when compared to the bun (the only real comparison possible), so that's clearly not the answer.

I would point to the scene where they're rolling the patty on the skillet to flatten the edges. That, combined with possibly less squishing by the press and less cooking overall would give you thicker edges in particular, making the patty look bigger in the ad. We only see the edge and overall width, after all, not the whole patty.