At the moment of separation of RSDRP mensheviks were in fact a minority of the party. And intersting enough, it's not the given name, that's what they named their faction after bolsheviks came up with their name.

They were minority nearly all time among the workers and party supporters in Russia. Mensheviks did attained majority in party congresses several times, but it's because they were usually happening outside Russia and mensheviks certainly had majority among the emmigrant inteligentsia. For example once they got a majority for few years because most bolshevik memebers of Central Comitte were arrested in Russia - something that was not happening to the mensheviks since they weren't in Russia.

They also didn't had the majority during the October Revolution. They did, but only together with SR's, during the February Revolution, but in subsequent months they squandered it by not fullfilling even the most basic promises of that revolution.