Which then translates to, "Citizens/Pop is power," no?
This is making me re-think the power of the Kymer UB, which basically reads "turn one plains into a grassland in every city."
It's even better than that when you think about speed of your population growth. The Khmer UB gives +1 food and that's not just an increased max population size, it's also a vast improvement on how fast you gain in population depending on the city.
Imagine a city that is entirely riverside grassland cottages. This is obviously going to be an immensely powerful commerce city eventually. You start out with a 2 food per turn surplus and as long as you don't hit the

or

caps, you'll keep a 2 food per turn surplus forever.
With the Khmer Aquaduct, you have a 3 food per turn surplus. In any particular turn, you are gaining food 50% faster than the other civs. You're growing population less than 50% faster than your counterparts because you presumeably have a greater population and therefore need more food to grow, but it's still at least 30% faster even taking that greater population into consideration.
Imagine you had to work a plains tile for some reason (perhaps you're growing the tile for the capital to work later). The Khmer building turns that 1 food per turn surplus into a 2 food per turn surplus. That's a 100% increase in per turn food accumulation.
I've noticed that cities which have only grasslands with either no access to fresh water or which have only cottages built are dismally slow to grow. Add in a food resource and suddenly you have a commerce powerhouse. That's because even though the max population that the city can support is not significantly greater between irrigated corn + grassland cottages vs. grassland cottages alone, the irrigated corn city is going to get those population points 3 or 4 times as fast as the city that builds only cottages.
To me, the formula is:
Improved land being worked is power.
Improved land being worked is power.
Improved land being worked is power.
...if you're not working it, what good is that land doing you? And if you haven't improved it, then you're not really working it.