Death rates are particularly hard to compare because part of the idea of El Salvador's system is that people are expected to die there - there is no release policy - yet most of them are young healthy men recently detained.

If we just look at incarceration rates:

CECOT is one facility, but around 2% of El Salvador's population has been imprisoned by Bukele's operation.

In 1950 the USSR had a population of around 180 million, and the gulag system was at its height with a population of 2.5 million, very similar.

The US prison system has been around 1% from the peak of the War On Drugs until recent fads in liberalized sentencing, currently holding at 0.7%, one of the highest in the world if you exclude ethnic purges like Xinjiang or Gaza.