From the video:

1) they stack up all pickles/onions/etc in the front for ads.

2) the real burger goes in a box and the steam makes the bun shrink a little

Also, they didn't say it but they showed it:

The burger is cooked differently for the ad. I got the impression they were just grilling it to make it look good but with minimal cooking (I wonder if it's even safe to eat the ad one). If you've ever cooked a burger, you'll know they shrink when you cook them.

Yeah, there was a kids' documentary made about fast-food photography a few years back. Basically they will just use a skillet to surface cook (brown) the outside of the patty to avoid the shrinkage that occurs when you grill a burger (lots of water and fat drip out and evaporate). This sort of answers the parent's question too. It's definitely not safe to eat.

IIRC, in the Burger King ads they heat thin metal strips and use them to sear lines into the patty, giving the "flame-grilled" illusion without having to cook and shrink the patty.

Actually, here's the children's documentary I mentioned. They also use a couple other sneaky tricks to make the product appear larger without image manipulation software.

http://www.youtube.com/watch?v=fUjz_eiIX8k

Further, the burger press used in store, is usually pushed on a bit by the employees to cook the burger faster, but also flattening it a bit, and getting rid of some more juices that make up volume.

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.

The bun doesn't shrink evenly. It strinks more under and above the ingredients - essentially sagging and hiding them.

I aggree that if it shrunk uniformly, then it would make the contents look proportionally bigger.

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.

I think we can infer that they didn't show all of the tricks they use. While it's true that all the ingredients have to be as sold in the store, there are plenty of tricks... they showed some, but possibly not all.

In the Metafilter thread, someone mentioned that on a styling shoot they'd been on, it appeared to be common practice the butterfly the back of the burger and manipulate it such that the patty appears thicker. I assume by front-loading the bulk of the patty toward the camera.

That may or may not have been done, but it's an example of a trick that abides by the "must be the same stuff as is sold" rule but would still result in a larger-looking patty. I think we can assume that not every little trick made it into the few-minute edit of the few-hour styling process, and likely the tricks selected for the final edit were the least deceptive.

Another small factor at play is selection bias. They have lots of buns and lots of patties to choose from, and they choose the ones that look the best.

They dont cook the burger for the pictures, just the edges and outside of it.

She says the retail burger looks smaller because the bun collapsed from the moisture in the enclosed box. She also describes how they arrange the burger to "lean" in the photo, so that might add to the effect. Also the obvious reason would be the one burger was slapped together in 45sec vs. 4-5 hours of meticulous grooming: Selecting the loftiest bun, searing the edges to square up the perimeter of the patty before grilling, delicately balancing the layers on top of each other, etc...

The perspective created by the arrangement of the buns is probably a big part of it.

I'm not actually sure the meat does look smaller if you lined up the photos to take the optical illusion out of it.

They do exactly that when they're showing the Photoshop workflow. The actual patty looks like it might be a hair thinner, but most of the difference is your perspective being thrown off by the collapsed bun and melted cheese from what I can see.

You're picking at the difference between the proximate cause and the ultimate cause of the ad looking the way it does.

    Q: Why does the ad burger patty appear thicker and larger?
    A: Because we made it *this* way [show process].

Sounds reasonable to me. Perhaps you wanted an answer more like,

    A: Because we wanted to show off all the ingredients in the
       best possible light, to encourage you to buy it.

It actually takes a few why/because cycles (i.e. root cause analysis) to approach the ultimate cause; for example:

    A: Because we made it *this* way.
    Q: Why?
    A: Because it makes the burger look good and it shows
       you what's in it.
    Q: Why?
    A: Because if the burger looks good and you know what's
       in it, you'd be more likely to buy it.
    Q: ???
    A: Profit!!!

Alas, the original question did not stipulate a particular kind of answer; I think the response, as given, sufficiently answers the question.

“Here you can definitely see there’s a size difference. The boxes that our sandwiches come in keep the sandwiches warm, which creates a bit of a steam effect, and it does make the bun contract a little bit.”

3:05 – 3:12

They addressed it directly. When it sits in the box, the steam makes the bun wilt.

Referring to the meat, not the bun.

McDonald's grills press the meat patties in real-world production. It drives out fluids and the finished product appears smaller than one cooked by different methods.

Which doesn't make the ads any less shammy. "Here is a representation of the product produced in a way that is nothing like the actual process to produce said product in an attempt to make the product look better than it actually is". Hmmm...

"We cooked this using a completely different process to make a more appealing image to sell you."

Again a good explanation, but it doesn't make it any less of a lie to show a larger patty.

It could very well be the same size patty, just nearly uncooked.

Oops—my response was unclear. I should have said "seemingly-larger" patty. My complaint is that they're cooked completely differently, leading to a final image where the patty appears larger than anything you would get from a McDonalds.

It's shown diagonally. Watch the video, the presenter mentions this.

The question they were asked was "why?" not "how?"

The question asked was, originally:

Why does the ad burger patty appear thicker and larger?

Which to mst people would be a question of appearance, not intent.

Their answer is because the box used to keep the burger warm generates steam which reduces the size of the bun.