Let's see, during Stalin's Rule 18 million people went through forced labor camps and roughly 10% died, around 1.8 million
Let's add around 5 million for man made famine, and probably a 2 million for arbitrary executions and deportations, while many estimate the full death count as between 15-20 million
As far I can understand the top range of estimates for CECOT, which is a non American facility, are that 500 died, of around imprisoned 20,000 inmates. So the scale is a bit... different
I think the issue here is that contrary to popular belief, not every wrong thing is the same
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.