Reddit karma uses a logarithmic function to determine the karma on a post. The first upvotes give 1 karma each, but the higher the number gets the less karma you get from additional upvotes.

This isn’t even some theory based on observations, the reddit algorithm used to be completely open source so you can see how it works on github (or at least how it used to work 7 years ago).

Front page reddit posts with 50k points most likely have over a million upvotes in reality