Maybe the other photographer isn’t so lazy as to worry about an extra couple pounds when shooting, so he uses a full size camera all the time.
LOL! Well, that may be. The difference between my Rebel, flash and a couple of lenses, a 10-20, 18-250 and a 50, plus batteries and other miscellaneous stuff compared to a 1Dx with flash, 16-35, 28-300 and an 85 plus batteries and other miscellaneous stuff is about 8 pounds. At my age, carrying that extra weight when I don’t need to is wearing.
There are times when the smaller body can deliver just as good results, or allows me to go somewhere I wouldn’t bother to go with the heavier bag. Also, there are times the bigger body and lens just attract too much attention. With the smaller body I’m not noticed as much. When I really don’t want to be noticed I take my wife’s P&S, then I am almost completely ignored. Sometimes getting the shot requires specialized gear, and sometimes that gear is a smaller camera.