They use different bun purveyors. West coast has sesame seeds, northeast has weird wheat bits and Canada has a smaller bun with some sesame seeds.

IMO, Canada has the superior bun. They also have the smoked meat sandwich, but their pizza is ass.

I've found the pizza varies a lot from location to location, within the same region. The good ones are some of the best pizza I've had, and others are less desirable than frozen pizza.

bun size may depend on location

It definitely does. As I just learned today, Costco stores source buns from different local/regional bakeries, so the size and quality can vary widely. I also learned that people have been complaining about the Costco hot dog buns getting smaller for at least a couple of years now.