I think what you're missing is that sample sizes for statistical significance do not depend on the population size. The big caveat is that the sample is random and representative, which you can't really verify beyond doubt. But if a sample is random and representative, you could estimate population statistics of millions (or even infinite) with a sample of just 30 or so.
As sibling posts mentioned, a much bigger issue than sample size is representativeness: college psychology study samples tend to be drawn disproportionately from college students. They might not generalize well to the general population even if you increase the sample size ten fold or a hundred fold.
As sibling posts mentioned, a much bigger issue than sample size is representativeness: college psychology study samples tend to be drawn disproportionately from college students. They might not generalize well to the general population even if you increase the sample size ten fold or a hundred fold.