They are huge, but they're also rare, and excessive. Especially for Chernobyl the exclusion zone includes vast areas no more radioactive than, say, Denver or other high altitude cities, and people kept working at Chernobyl (several of the reactors remained operational) for many years, and people have kept living within the exclusion zones. It was probably the right thing to set it as big as it was given we didn't know better at the time, but if anything Chernobyl has shown that the effects are far less severe than feared.