Microwave oven uses magnetron to generate hugh power em waves. How the power output of a microwave can be increased? It uses a 1-2 KVAC capacitor. Will doubling the voltage of capacitor double the power output?
You will have to change the magnetron and change the capacitor for a higher capacitance. Most lower wattage microwaves have lower capacitance capacitors. Or you can ditch the Microwave oven transformer and capacitor idea together. You can replace it with a panasonic DC to AC high voltage Power Converter. Those are more efficient and are usually higher wattage. Most of the time 1200 watts. But be careful when changing stuff in a microwave, because it can kill you! Always disconnect the cord and discharge the High Voltage Capacitor if your microwave has one! Consider to either by a new one or ask a trained electrician to to it.