It may not only be a function of the size of the group, but also the size and type of schools/systems where they tend to have chapters. Being in the NPC at some schools is a hindrance, since locals have fewer regulations and can recruit according to the campus culture.
__________________ Adding 's does not make a word, not even an acronym, plural
Last time time I talked to NIC, thery had no desire to expand.
When I aked why, they said so many new groups sprouting up, Latino, Asian, Gay that they would not be interested.
I would assume the same for PNC, who are more strict than NIC.