I'm really glad I recently replaced my old 80% efficient oil boiler with a 97% efficient natural gas boiler. My winter bills are less than half what they were and it barely uses any gas during the summer for hot water.
So if fuel prices and the amount of useful heat usage stayed the same, then your bill should be 1.03/1.25≈82% of what it was before. If your bill is "less than half" then something else must have changed.
Any modern boiler (gas or oil) will almost certainly be cold start, so it only fires when there's a call for heat. This alone probably saves a gallon of oil a day at least.