They can be correlated without being a tight linear relationship.

If every two people in a group have exactly one transaction, then adding 20 people to a group of 100 will create more new transactions than you'll find in a group with just 20 people.

To be fair, when I google for research on correlations between economic growth and population growth, I get a jumble of papers on opposite sides - some pro-natalist and some anti-natalist and some agnostic. (Maybe one camp is considered far more authoritative than the others?)

Maybe it's a nontrivial relationship if it exists, though I'm not convinced anyone knows definitively how the relationship plays out.

The fact that the slope of the growth line is not 1 does not imply that economic expansion is not tightly bound to population expansion, or that economic expansion could continue under population contraction. Consider the simplest possible linear model Y = beta * X + noise where beta != 1; to say nothing of nonlinear models or interaction effects between population growth and other factors. Surely output per capita matters, and the true data generating process is far more complex, but you have not come close to demonstrating population growth and economic growth are decoupled through this example.

It's easy to grow fast while borrowing heavily, and that should be taken into account when looking at growth curves for various economic regions. Oil prices have also dropped from around $100 to $60 iirc.

2008-2018: total US debt (government, corporate, household, non-profit orgs) increased almost 9% from ca $68.1T to $74.1T, if you trust the numbers (https://www.nwcapitalsolutions.com/project/us-deleveraging-a...)

I wonder if the effects of automation, which the previous commenter alluded to, contributed significantly to this phenomenon...