Do you ever think about how much attention and thought they put into the way they stand together? It's a simple perception trick.

From a very early time period, to present day, people have wondered about the way Gulf appears smaller, and a lot of times it's this perception trick at work.

Gulf very regularly stands, or leans, at least slightly behind Mew. Doing this makes him further from the camera so he appears slightly smaller. Him slouching if he's in front helps too.

You can see the visual difference here. it also helps that his frame is quite a bit narrower than Mew's is.

One of the other notable tricks is turning his body to the side to appear slimmer, it aids in the visual difference.

Whether purposeful, or not, I just find it cute how they constantly manage to make Gulf the smaller of the two, even though they're much more equally matched than they sometimes appear.