Why are amplifier transformers sized the way they

Are you talking about power transformers, or output transformers of tube amps?

A large power transformer will stay reasonably cool when the amplifier is driven hard. It also will continue to deliver full voltage under load. A large transformer benefits continuous audio power capability. Peak power demands should be met by the power supply capacitors, with the transformer having lesser importance.

A large audio transformer implies use of heavy gage wire in the windings, and plenty of iron in the magnetic core. This avoids magnetic saturation at high power, thus reducing distortion.

Running a very large transformer reduces the potential for distortion, provides a lower impedance path for the AC, can deliver more current as needed in a more timely basis i.e. improved transient response under load, etc... There are no drawbacks to using a larger transformer than what you need other than the added cost and weight involved.

On the other hand, using too small of a transformer can result in reduced power potential, increased distortion, poorer transient response under heavy load, etc...

There was an old saying pertaining to amplifiers. That is, the heavier the amp, the better it was built. In many cases, this is the truth. Then again, one can skimp on a transformer and heatsinking and use double wall thickness chassis parts and achieve equal or greater mass. If you took the fancy "sculptured" faceplate off of some amps, it would cut their weight down quite measurably. Sean
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Sean, I have a quick question for you, if you don't mind. I need to buy a sub panel box. I am confused as to which brand/model/features that I should look for. My current thought is just about any that has a solid copper bus. Is there anything else I should know? Thanks, Brooks