Perhaps this is a bit of a Golidlocks effect. Small groups work when they are not too small and not too big.
"Twelve appears to be a magic number of members for the SMALL GROUP. Although The Cambridge Apostles (so named because, like Jesus’ followers, there were twelve of them) were one of the few groups that stuck explicitly to this number, The Junto Club, The Inklings and The Bloomsbury Group all had approximately twelve core members. Homebrew is the only real outlier with about 750 members, although it was still small enough in practice for members to meet in one another's garages."
Around a dozen members is the sweet spot of social motivation: small enough to know everyone, yet large enough that the group won’t collapse if one or two members’ enthusiasm wanes; small enough that you are not daunted by competing with the whole world, yet large enough that you still need to be on your toes to keep up.
Of course, there really is no one single way for any kind of group success, but maybe you can ask yourself questions like:
is this group the right size for what we are trying to achieve?
are problems occurring which relate to the size of the group?
are there signs it is too small?
are there signs that it is too big?
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