Good answer – even though the question is a bit misleading. To put it in concrete physical terms, any particular transformer design is limited in power output by its ability to effectively shed heat. Continuously overload a transformer of any size, and it will ultimately burn up through failure of its insulation system. Within the limits of any fixed size of transformer, there is a finite, fixed limit of allowable power loss. Increasing output voltage means increasing the number of secondary turns. With a given size core, to physically fit more turns on the secondary, the size of the secondary wire must be reduced. The additional number of turns also increases the total resistive length of the secondary winding. More turns / smaller wire = higher secondary resistance = lower output current at the fixed limit of allowable secondary power loss.
