How were Mao Zedong and Stalin mass murderers?

Hello, I'm doing a report on Chairman Mao Zedong and my friend is doing Josef Stalin I've seen a few sites that says Mao Zedong committed the worst genocide in history with an estimated 100 people killed because of him, while Stalin is second with 75 million. I'm not sure if I'm looking at the right sources because I don't know how they killed that many people. Can somebody shed light on how this two caused genocides?