If you rectify 30VAC then you get a 42.3V peak voltage. The full-wave rectifier drops 2V and the filter capacitor output is 40.4VDC. When the current is 10A then the transformer must supply 423VA, not 300VA.
If the output is 30VDC at 10A then the load heats with 300W and the rectifier and voltage regulator heat with (42.3V - 30V) x 10A= 123W.
Some people tear down the transformer from a microwave oven. It is powerful but its full power rating is only for a couple of minutes. It will overheat if it runs a high power continuously. Maybe its magnetron fan also tries to cool the transformer.