Yes, they were making them bow their heads.
There are a lot of “western” born jihadists in ISIS who would likely be much larger than the average poor Egyptian laborer...
I noticed the size difference, too. While it’s true that impoverished Egyptian laborers are likely to be smaller, I suspect they had also done something with the camerawork. These videos are very slick (supposedly, the “production company” is run by an American or at least, some radical Muslim who once was an American citizen). You’ll notice that the jihadis are actually walking on a slightly higher part of the beach, in a line in such a way that it would probably be possible to magnify their size with the camera.
It was so uniform throughout the vid that it was clearly done intentionally and for effect.