Furthermore, whatever company is doing the shoot will use their best camera for whichever project. If they have one of each, they’ll often have the smaller camera capturing the same events from a second angle, or getting behind-the-scenes footage. I’ve been that BTS guy holding the smaller camera. For BTS, the lightness of the smaller camera makes it easier to catch unexpected BTS content, if you have to swivel around or zoom in on the fly, for example. No one cares if BTS footage is shaky.