I recently visited a railway museum with a lot of Soviet-era locomotives. The picture below shows the wheels on a locomotive from the 1930s or 1940s. Some of the wheels are thicker on one side (as it it were a partially filled cup). What is the purpose of that?

[![Picture of the wheels of a Soviet-era locomotive](https://i.stack.imgur.com/JbStW.jpg)](https://i.stack.imgur.com/JbStW.jpg)

(Now that I am looking at the picture again I wonder if they are balancing the weight of the coupling rods in some way.)