Check out these side by side shots of guardians in regular aspect ration and IMAX aspect ratio.
James Gunn personally chose all of the scenes for the difference:
We will be changing aspect ratios throughout the film in a way that makes the experience even fuller and more encompassing. I’ve personally chosen all the places where the changes occur and, again, I love how it’s coming along. The changing aspect ratios in this case are actually a part of the storytelling.”