This isn't directly related to medicine, but it isn't exactly irrelevant either. It relates to the Hardy Weinberg equilibrium, but the logic behind it is interesting.
The answer is that a small population evolves quicker. Basically, this is because random events have a disproportionately large effect on shaping small populations, whereas big populations are largely insulated from its effects.
Let me explain by means of an example. Consider how rapidly a population could change if it only had 6 individuals, and the one with a rare gene (allele) fell off a cliff. The next generation would have 'evolved', as no one would have this gene anymore. Or consider the converse - in a population of 6 I hold a rare allele, but a freak lightning strike wipes out all the holders of the 'common' allele. Suddenly, the 'rare' allele becomes proportionately more common! This is an extreme (and a little bit silly) example of the phenomenon known as genetic drift - allele frequencies are changed by random chance, rather than by natural selection.
This effect is much harder to match with a large population - say of a million individuals. For a random event to wipe out, by sheer accident, all (or even most) of the holders of a rare allele is clearly much harder if here are tens of thousands of them. In other words, genetic drift has a smaller impact on a larger population. Therefore, the alleles in a small population tend to change more rapidly, which is to say that such a population evolves more quickly.
Furthermore, although it isn't often mentioned in textbooks, a mutation (as distinct from the random sampling error of genetic drift) will make its presence felt slightly more strongly, all things being equal, if the population is small (consider: a new allele out of six, or out of a million - the small population would show the effects more quickly).