I'm not speaking for RJ but there's probably a couple different reasons that one could limit the coop, by keeping the numbers lower, it's easier to get a quantity of an item, (say from mouser) than it would to get say 1000, as opposed to 300.
also, RJ, has his dad helping him, He's fast but I'm sure he doesn't want to stress the guy out.
also by doing the small quick ones, there's probably less worry over did everyone get covered. Not to mention, RJ usually programs the pics for us, so that takes time as well.
He does several Express coops throughout the year. So if you don't get all you want, then jump in early on the next one.
I've jumped in a couple last year. I'm not finished with my purchases, but I've sat out the past couple (including this one) so that other people (especially new guys) can get in on having one.