Max Tegmark observed that we have three independent reasons to believe we live in a Big World:  A universe which is large relative to the space of possibilities.  For example, on current physics, the universe appears to be spatially infinite (though I'm not clear on how strongly this is implied by the standard model).

If the universe is spatially infinite, then, on average, we should expect that no more than 10^10^29 meters away is an exact duplicate of you.  If you're looking for an exact duplicate of a Hubble volume - an object the size of our observable universe - then you should still on average only need to look 10^10^115 lightyears.  (These are numbers based on a highly conservative counting of "physically possible" states, e.g. packing the whole Hubble volume with potential protons at maximum density given by the Pauli Exclusion principle, and then allowing each proton to be present or absent.)

The most popular cosmological theories also call for an "inflationary" scenario in which many different universes would be eternally budding off, our own universe being only one bud.  And finally there are the alternative decoherent branches of the grand quantum distribution, aka "many worlds", whose presence is unambiguously implied by the simplest mathematics that fits our quantum experiments.

Ever since I realized that physics seems to tell us straight out that we live in a Big World, I've become much less focused on creating lots of people, and much more focused on ensuring the welfare of people who are already alive.