Fundamentally, the "size of the economy" measures value creation, i.e. the conversion of low value atoms into higher value atoms. The economy grows by either c

Fundamentally, the "size of the economy" measures value creation, i.e. the conversion of low value atoms into higher value atoms. The economy grows by either converting more atoms, or changing the value of the in/output. Obviously "sustaining more people" requires proportionally more atoms, so exponentially growing population implies exponential economic growth. What happens in the constant population assumption is an open question. It could be that our rate of atom conversion also becomes constant/decline. We might become more efficient and grow the economy by increasing the value of the atoms we convert. The economy might cease growing altogether because we reach a technological plateau and need satiety. But it's also possible that we'll decouple demand from the capitas, and find whole new ways and reasons to increase atom conversion.