Fishermen and guys holding antlers do it all the time... It makes the trophy look bigger. BUT, my opinion is that it isn't intentional on their part to deceive most of the time. What's the point of the picture? To show off the FISH! So why not hold it out so the fish is the focal point, while they get a good shot of the fisherman in the background... The intent isn't to "super size" the fish, but to show it off.

The same thing happens to a big deer, elk, or any other "trophy" that's getting its pic taken. If I'm showing you a picture of me with my big whitetail, you already know what I look like... Let's see the deer!