More food means more population and more population means more production. Don't see what's wrong.

the layout of that planet is a bit inefficient, but the farms aren't that bad actually. would have been smarter to put the farms next to the hospital to actually get some more growth. but overall that layout is already much better than what they did 2 patches ago

they also built 2 approval buildings, so their strategy was probably to grow a big population on that planet. the rules regarding prodction and population were recently changed (again) and high population is now more efficient than before. it's a valid strategy. maybe not the most efficient, but i've seen them do worse.

... it's still going to have 3 farms in the end to hit 20 bil. Why not get them built early and avoid wasting turns of valuable growth?

Because building them before you come close to the pre-farm population cap delays building much more immediately valuable structures. Furthermore, even if your timing is somewhat off, a planet with a decent manufacturing level can fairly easily make up for the lost turns of growth; it's only 0.1 population growth per turn as a base value, modified by approval, colonial hospitals, other growth bonuses, and birthing subsidies, which, except on a high-manufacturing world, probably won't add up to more than 0.2-0.3 population growth per turn.

Checking against the in-game numbers, it appears that population is not capped at 30, nor is the production bonus from population capped at 30. I don't know what PlanetFullPopulation in GalCiv3GlobalDefs.XML is supposed to do, but it's either not supposed to cap the population or population-based production, or it's not working.

It is exactly what i get, insane amount of farms when population is really low.