No they were the majority in 1917. Earlier they had been the minority, but post Kornilov they were a majority at the time of the storming of the winter palace.
The Bolsheviks, was a faction founded by Vladimir Lenin that split from the Menshevik faction of the Marxist Russian Social Democratic Labour Party (RSDLP) at its Second Party Congress in 1903. https://en.wikipedia.org/wiki/Bolsheviks