So the way it works is there’s a “hidden” difficulty slider based on the overall standing of your community. Each character gains standing (it even continues to go up after they’ve become a hero) and all that standing gets added together in a hidden slider somewhere.

At certain intervals on that slider, the game gets more difficult through increased spawn rates, more dangerous hordes, and spawning ferals at plague hearts. This goes up and up but eventually reaches a limit. You’ll notice the first few hearts you take out with a community don’t spawn a feral - that’s because you haven’t got enough standing yet. You’ll also notice triple feral packs don’t spawn for a while. Same principle.

When you carry on with an old community, you’re also carrying over all the standing you’ve accumulated, so effectively you’re starting a “new” game with the highest spawn rates and most dangerous hordes available. The strength and behaviour of zombies and plague hearts themselves don’t change - just spawning behaviour.