I wonder if it has to do with the predictability of large vs small groups.

If you only have 30 people to interact with, they could collectively change their minds on a whim. It's harder for 30 people to reliably uphold a system of expectations, because it's a small enough number, that they can change the system quickly.

For example, if you try to establish a monetary currency among 30 people, it will be kind of obvious that the value of the currency is just made up, and you're all kind of playing a game together. But the game only lasts as long as the 30 people still believe in it. And they may choose to ignore it if it no longer suits their needs.

But if 1 million people all use a common currency, then it will feel more real and permanent. Even if your small social community of 30 people decides they don't want to honor the currency anymore, well, you can always rely on the rest of the 1 million people to honor it.

We tend to think of trust, or trustworthiness, as a virtuous personality trait. But I think when push comes to shove, trust has less to do with virtue, and more to do with predictability.