That I'm not sure. I only recalled reading how franchise stores basically have more freedom, and they have an option to choose their own source (for example chicken) as long as they use the same recipe and pass HQ's quality tests.
Since then, whenever the chicken is smaller, I assume it's a franchise store that decided to source independently. Because the ones that come directly from JFC are from big chickens.
I guess the best way is to ask.
Hmm… that gave me an idea, track and mark branches that are franchises and have small chickens. ^_^;;