John wrote

Just curious (not clever enough to work it out) but why are
3 phase chargers more powerful than single phase fed ones?

For the same reason that 3 phase motors
are more powerful than single phase motors.
The higher current is supplied over 3 phases.

Its actually more complicated than that
with motors, but not with chargers.

Is it just that the mains circuits have different capacities

Yep.

or is a 3 phase when rectified more powerful?

Nope.