Giants touch themselves!
I always wondered why even capitals have a variable pop. I think it's mostly aesthetics, so that they're not always pegged at exactly 30000. Though, it seems to me that the variance is a little wide, I think it can go anywhere from 29000-31000, which is a decent chunk of people, especially if you DO have a Growth or Death scale that is going to compound directly from that value to the end of the game.
It takes ~11 turns to get from 29k-31k. So, one year of game time worth of your Growth can significantly impact the actual benefit of a strategy involving that scale.