Where is your property located, and does the ground stay frozen most of the winter? If so, the trick is to establish a plow base that protects the gravel, and you only ever plow snow down to that base, not the gravel. I can't do that where I live though, since the ground doesn't stay frozen for more than a few weeks in the peak of winter. So I just have to be careful and keep the plow edge up off the gravel a little. I plan to put a rubber edge on my blade this winter to be even less destructive to the gravel.
If the plow driver is widening the driveway by several feet with all the gravel, it sounds like his shoes are set so that the cutting edge is all the way down, and he's moving a serious amount of gravel off to the sides with the snow. Not much can be done about that unless he adjusts his shoes and habits. Probably too much to ask if this guy does lots of driveways, especially paved ones where owners probably expect him to scrape down to the surface.
Another idea is to just let the driveway get wider but still only have him plow the middle strip. Then in spring or summer, you can use a rear blade on a tractor to re-crown the driveway, by angling the blade to move gravel from sides back to center. So you will have a permanently wider driveway, but do an annual re-crowning in the spring/summer so that it doesn't keep getting wider and wider every winter. Now that I think about it, that would probably be my compromise solution.