In my opinion bigger is not always better although I know I'm in the minority on this opinion.
If the majority of your cooks are for a limited number of people the smaller kamado can just be easier and faster. With a smaller kamado you might have to alter the menu a bit for larger gatherings but that's easy enough to accomplish, just takes a little more planning.
Mostly I cook for 3 people on a kamado almost daily. I went the other extreme and got the 16.5". It quick, easy and uses a lot less lump.