Microwaves are, unsurprisingly, waves and act accordingly.
When waves are created, they travel away from the source until they are either absorbed or reflected. This is known as forward power. A closed cavity reflector such as a metal oven absorbs very little, but reflects them around until they return to the source. This is known as reflected power.
At some point the wave power in the closed box must be absorbed, causing local heating. If the oven is empty, most of the power will be re-absorbed by the emitting circuitry, causing it to overheat. Depending on the technology the oven will either turn itself off briefly, turn itself down, or break down.
Food is the obvious absorber, or load. The maximum power theorem states that the maximum amount of power will be absorbed when the source and load impedances are equal, or matched. The oven will have a design limit on the amount of food it can cook and will (or should) be designed to match the impedances at this point.
Any different amount of food will cause some power to be reflected and the source must be capable of absorbing that reflected power. This is as true for excessive amounts of food as it is for small amounts. In practice food is not closely-characterised electromagnetically and there will almost always be 20% or more reflected power to deal with.
As well as the power absorbed by the food, the total power emitted includes any reflected component.
The power actually consumed also depends on inefficiencies such as the internal resistance of the circuit. The "power" charged for by the supplier will also depend on the power factor of the circuit in operation, as suppliers actually charge for current not power; this is true for all electrical appliances.