It took centuries for the slave trade to approximate the death toll of communist regimes. Fascism and national socialism were also utopian projects. Under all of these projects, millions of people were enslaved, imprisoned, persecuted, and murdered in vast numbers. I’m not saying the aggregate amount of human misery is the same, but the amount of misery inflicted in a given year was dramatically higher.
“Utopian” is used in this context to refer to efforts to rebuild society from the ground up on logical principles chosen in isolation.
That's mostly because we were billions of people when the 20th century utopian projects started, something we've only been for the past 200 years or so.
The population of Ancient Egypt was about 1 million at the start of recorded history and about 5 million during the Roman conquest. Yet, even though the number of Egyptians in slavery were only in the millions at its highest it was crime that was very widespread in relation to the population.
How many Russians were effectively living in slavery under Stalin? Or Cambodians under Pol Pot? All of them? Certainly a very high proportion.
1/3 of the world’s Jewish population was wiped out in a decade. At least 1/10th of the population of Ukraine starved in the two year Holodomor. Double-digit percentages of the entire populations of the Soviet Union, Poland, Greece, and Yugoslavia perished in the Second World War as did about 8% of all Germans. 1/4 of Cambodians died at the hands of the Khmer Rouge. All within brief periods of time.
It's estimated around 80% of the population of Egypt at any time was in some sort of slavery, whether chattel slavery, bond slavery or through forced labor the latter of which is what I would stretch including the citizens of the USSR into.
“Utopian” is used in this context to refer to efforts to rebuild society from the ground up on logical principles chosen in isolation.