Surprisingly, not that big! Unlike a few decades ago, when most physical slots were mechanical, today the overwhelming majority of them are digital, meaning that, at least in theory, physical and online slot games are exactly the same. Of course, that isn't true for every slot, many of them have separate online and physical versions (like Bejeweled by IGT, which is a completely different slot depending on where you play it), but ultimately they're both just software running on a computer.