The best strategy for getting away with lying is to lie small by only deviating from the truth as much as is necessary to achieve the intended deception. Why then do some demagogues lie big? One set of views has it that the only difference between small and big lies concerns the size of their contents. They claim that the purpose of big lies is the formation of false beliefs in their literal contents via counterfactual reasoning, conspiracy theories, or the illusory truth effect. The negative part of this paper questions these accounts. The positive part proposes a different explanation for why demagogues use big lies and argues that big lies may serve three distinct purposes for demagogues: they reinforce their supporters’ deeply held beliefs, test the loyalty of their close followers, or publicly demonstrate the demagogue’s power. For a big lie to serve these purposes, genuine belief in the lie is not required – in fact, few are likely to believe it. What matters is that the demagogue’s supporters publicly endorse the lie. We contend that they do so, either because they interpret them as motivational statements or use them to express or justify their shared emotions or convictions.