The transformer may be "on" all the time, but it is not fully loaded all the time.
If the actual connected loads were 300 amps, and the transformer rated at 361 amps ..... your load on the transformer is only 300 amps.
Now, if the connected loads were a continuous 300 amps, I would want the wires to be able to handle 300 x 1.25, or 375 amps.
It's the actual loads that matter - except that the transformer feeders cannot be smaller than the 362 amps called for by the transformers nameplate.
