I've actually heard estimates that Stalin killed more of his own people than Hitler killed in the Holocaust. There was some incident relatively recently where they found a mass grave that had been hidden and managed to stay secret until they started digging up bones.

Soviets did target minority nations for persecution (though the Jews had it bad even in Tsarist Russia--watch Fiddler on the Roof) but the real target were disloyalists, a term I've just coined for people who couldn't follow the party line enough. But our awareness of all the people killed was pretty late.

However, I do not believe any of the machine of fear and informing and KGB torture and disappearing quite compares to the sheer slick religion of twisted hate that was Nazism. Quality over quantity IMHO.