The size of the group is not as important as the committment and skill of the group members. The Mafia "families" were relatively small yet yielded enormous power. I'm curious as to the size of the "Communist" party at the time of the Russian revolution. How many "Bolsheviks" were there really there who took over a nation of perhaps 150million people? The group was I think relatively small but unbelievable effective at taking power (and abusing it.)

Another, but perhaps completely different, "revolutionary" group would be the Quakers in the USA. They have always been a pretty small religious sect, but they have apparently had enormous influence in reforms in the American social contract. I believe they were key players in both the abolition of slavery, the civil rights movement of the 50s and 60s and the Viet Nam anti-war movement. Of course they may not have been very effective in any of those endeavors. The Viet Nam war went on for long after the anti war movement started, slavery did not end in the South (it just stopped calling itself slavery) and look at civil rights in the USA today. Were their efforts, though high profile, effective?

Wild Bill