Well, you need to put the numbers in perspective. In 1789 the population of Paris was around 600-700k. So in relative terms, an amount of people that is equivalent to 6%ish of Paris got executed, and in total a fifth of the equivalent of Paris died. Seems pretty bad to me. Also, all this happened in 3 short years.
Besides, they're both just really horrific. "40k is nothing" is not quite the right attitude here.
- Over a 5 year period, ~40k is ~8k/year or around 22/day for the French revolution, with a population of ~26M around 1790.
- Over a 29 year period, ~20M is ~700k/year or around 1.7k/day for Staline, with a population of around ~102M around 1950.
Not discounting the horrific nature of getting your head chopped off of course. But all things equal the guillotine and arbitrary executions during the French revolution was nowhere near as deadly as Staline or other 20th-century mass-murder regimes. Humanity made great "progress" in between the two in its ability to industrialize murder.
Besides, they're both just really horrific. "40k is nothing" is not quite the right attitude here.