Why?

According to Andrew Stuttaford:

From the WSJ’s interview with Rick Santorum:

Mr. Santorum also believes that making babies creates wealth. It’s very difficult to grow an economy with a shrinking population, he says, pointing to the “demographic winter in Europe” as a cause of that region’s troubles.

This is somewhat akin to arguing that the United States risks starvation because there are fewer farm workers than there once were.  Times and productivity have moved on. The belief that rising per capita income (the number that counts: the overall size of the economy is a relative irrelevance) depends on population increase is an idea that technology and globalization ought to have left behind.

Why? What astounding piece of technology has made people obsolete? (If you answer “Watson,” I've already considered that.) I suspect that the older arguments for slow arguments for slow population growth are becoming less relevant. Capital investment is still important but capital can multiply faster when it consists of bits instead of atoms. Globalization might seem more relevant on the grounds that local population is less important than it used to be … but it also makes global population more important.

But wait, there's more. We also see:

Indications (pre-dating the 08/09 slump) that the American economy has been finding it increasingly difficult to generate jobs at a rate that keeps pace with the country’s growing population are a warning sign of further difficulties to come.

The “economy” is not a single Huge Thing with a limited number of jobs it can generate. It consists of human beings. (It's possible that the decline in question is due to a decline in the startup demographic.)

I won't more than mention the possibility that the total population is the important number and per capita income is merely a means to that end.