There is approximately 111m ethnic Russians living in Russia, out of a total population of 146m; with up to 150m Russians living in the world.   Russians are by far the largest Slavic ethnic group.  

So how did they become so populous in comparison to other slavic groups?

Were they historically the most populous slavic group?

Surely there is some ethnic groups that have been slavicised along the way in that large Russian ethnic population.