Target audience, the smaller the better?

Hi,
I'm new to FB ads, so when I chose target audience (older women) and got 320,000 people,
I got warning "Your target audience selection is fair."
However, when I add some more interests (health), I got 8,400 people
and then it was OK, "Your target audience is defined."
I don't get it. Since I pay per click, not impression, I don't see why wouldn't
I have a wider audience? I mean, if they show ads to older women with health interests,
it's fine. But maybe many other older women (not…
Target audience, the smaller the better?

See original post by imbrod