Then they ask, "So, what's your hobby?" "30 cm", replies the worker and then sees that the commie party worker's turned all red. "In diameter!", he corrects.
LOL! The humor doesn't work very well in translation, because the joke is that the uneducated worker doesn't know the "trendy foreign word" хобби, and has misunderstood it as a slang term for the penis -- thus he answers with a boast about its size. (Russian has several rude words for the penis that all begin with the letter х. The mildest of these is probably хрен, which is semi-euphemistic and literally means "horseradish root".)

An approximately similar joke in English would be to have a professor of logic using the word "fallacy," and an uneducated person misunderstands the word as having something to do with "phallus" or "fellatio."

Or, one could make a joke about a pair of Russian doctors visiting an experimental medical facility at Caltech or MIT or wherever, and they see an injured soldier being fitted with a state-of-the-art "bionic" arm to replace the one he lost in Iraq.
The first Russian says to the second Russian: Ну, как? ("Well, what do you think of that?"), and the American soldier, thinking the question was addressed to him, answers: "No, it's the same old one I've always had!"