Just my hypothesis, but I wonder if larger sample sizes provide a more diverse population.

A study with 1000 individuals is likely a poor representation of a species of 8.2 billion. I understand that studies try to their best to use a diverse population, but I often question how successful many studies are at this endeavor.

use a diverse population

If that's the case, we should question whether different homogeneous population groups respond differently to the substance under test. After all, we don't want to know the "average temperature of patients in a hospital", do we?