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.
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.