Gym appearance

I noticed that some gyms are taller and have a rectangular shaped structure on top of them. At first i assumed that this meant the gym is full with 6 pokémon. However, I then noticed this is not the case and that some of these are not full. Additionally, I noticed some of the gyms without this taller appearance/structure were full with 6 pokémon. It also doesn't seem to be influence by whether or not there is a raid upcoming/ongoing. So does anyone know why some gyms have this appearance?