That's intuitively plausible, and in fact I think it's likely to be true, but as it happens it's also a testable proposition. Voter turnout varies quite a bit among modern democracies: for some voting is mandatory, for others it's optional, and levels of enforcement vary among polities with mandatory voting. Do the dominant parties within high-turnout polities tend to be more moderate relative to the polity's baseline?

Unfortunately you also need to control for architecture -- first-past-the-post election systems, for example, are often thought to have polarizing effects. That makes testing a lot harder than it'd otherwise be, and scopes it out of my relatively modest familiarity with different political systems. But it should be feasible in principle.