Say you have a population of 50 people that all live in the same town and all of them are in M/F couples (so 26 couples) and over a year every single one of them have a baby, so 26 babies.

Now you have a total town population of 76.

Now lets take that exact same town but there's 100 people living in it. Same deal, all of them are in couples, but this time only half of the couples have children (so 50 couples, 25 children) now there's a total population of 125 in the town.

The first town has a much higher birth rate, but the second town is more crowded.