I'm running multiple postgres installs (albeit small ones) in the default config.

Also, the default config of most databases is very conservative: You won't get speed, but you will get safety and it'll run on whatever hardware you throw at it.

Mongo on the other hand came with unsafe defaults and no obvious warning for people to change it. Additionally, after fixing the configuration, you would lose a lot of performance to the point of it being impractical to run mongo in a safe configuration.

Majority of installs or majority of transactions across all installs? I suspect that the majority of installs are indeed running default configuration, or at least close enough to default that the user wouldn't have looked closely enough to spot the kind of configuration issue being discussed.

Larger installs, which probably do account for the bulk of transactions, are more likely to have been tuned for speed, although probably keeping all of the safety settings at (or above) their defaults. But part of the point here is that in a safe configuration Mongo makes things slower than default.