Karma is in some sense literally true, but it's more obviously true in smaller contexts.

Your actions have indirect effects that later come back and affect you, too.

The quicker your actions affect you, the more obvious the indirect feedback loops.

If it's not obvious, then it's an "externality", and you don't think about it.

In a small community, the actions reflect back on you quickly.

If you do something shameful, the gossip will spread quickly and affect your ability to get things done in other contexts.

In a large, anonymous city, the social network density isn't sufficient; your bad actions are less likely to immediately affect you.

Indirect effects can happen in time or in space.

Over sufficiently long time horizons, your actions will affect you in some (possibly minor) way.

When the context is larger, it takes longer for the loops to close.

And the longer for the loop to close, the less obvious it is that it's a loop, because the effect has diffused in time and space and is less concentrated.