Transformer VA rating is largely dependent on the wire size inside the transformer and the temperature rise before it exceeds the UL insulation class rating. Most all common transformers use UL Class B insulation systems indicating they are good for up to 130C temperature rise. The large the copper wire diameter, the smaller the loss involved due to IR heating of the winding wires. Thus if you are not hammering your amp on a continuous basis, you will not hit this rating limit and thus be able to enjoy periods of higher power output. The windings have thermal inertia so they do not instantly heat the entire winding mass up on periods of peak power, similar to your soldering iron that takes some time to heat up.
