As more voters take advantage of the opportunity to cast ballots before Election Day, it seems reasonable to expect that turnout will grow as well, right?

Not necessarily, according to those who've studied it. The evidence of early voting's effect on turnout is actually quite mixed. Early voting can energize turnout, but it depends on the kind of election, the form of early voting available (vote-by-mail, "no-excuse" absentee, early-in-person, etc.) and a state's registration laws.