A researcher can keep the sample size low if the population is

When the population is homogeneous its units are alike, so a small sample can still represent it well.

A heterogeneous population has varied units and needs a larger sample to capture the variation.

An inaccessible population raises practical difficulty but does not by itself justify a small sample.

A very large population usually calls for a larger, not smaller, sample for the same precision.

Homogeneity lowers variance, which reduces the sample size needed for a given accuracy.

Required sample size rises with population variability and with the precision demanded.