Oversize is a relative term. If a preamp has a high-power amp's supply, then yes, based on power it would be overkill.
As an EE that specializes in power supply design, my personal opinion is that a large power supply is a good thing. The reason is that large transformers have a low internal impedance which allows them to pass current very quickly to the circuit. This speed and availability is what makes them better than a small transformer with thin wire.
Sometimes, sounding "dynamic" is due to a lack of bass. So yes, a small power supply can sound more "dynamic" than a large one - but that is simply because tonal balance is relative to itself and not because of power supply design. The two should not be confused.
Arthur
As an EE that specializes in power supply design, my personal opinion is that a large power supply is a good thing. The reason is that large transformers have a low internal impedance which allows them to pass current very quickly to the circuit. This speed and availability is what makes them better than a small transformer with thin wire.
Sometimes, sounding "dynamic" is due to a lack of bass. So yes, a small power supply can sound more "dynamic" than a large one - but that is simply because tonal balance is relative to itself and not because of power supply design. The two should not be confused.
Arthur