It is certainly true that larger networking groups can generate more referrals exponentially. True, that is, within each type of referral group. Among strong-contact referral groups, such as BNI, studies consistently show that a group with 40 members will typically generate more referrals per member than one with 25 members. It stands to reason: The more people in your group, the more Rolodexes you have access to. But the same thing happens among casual-contact networks like chambers of commerce; a 200-member group will probably generate more referrals than a 100-member group.
However this doesn't imply that a 100-member chamber of commerce will pass more referrals than a 40-member referral-networking organization. The strong-contact group focuses primarily on generating referrals for its members and is structured so that there's time for passing information and referrals in each meeting, and members are personally accountable for generating referrals for other members of the group. A chamber of commerce offers plenty of opportunities to pass referrals, including forming some special committees that can serve as a strong-contact referral group. This type of organization is really made up of three parts: It is part information network, part service organization, and part referral group.
Bear in mind that a master networker does not need solely a high-structured organization to generate and receive referrals. She can do this in almost any setting because she has highly developed relationship skills. She constantly looks for ways to help or benefit her networking partners, and she has a reputation as someone who can get things done, no matter what the organization or situation. For her, a casual-contact group can serve her just as well as a strong-contact group--perhaps better, because there are more possible connections in a larger group, whether structured to make those connections automatically or not.