The size of a microwave does not affect its performance. For example, larger microwaves do not take longer to cook food than smaller microwaves. A basic understanding of how microwaves work demonstrates that size is irrelevant to the microwave cooking method.
Microwaves work by a device inside the microwave called a magnetron, which sends out radio waves. These radio waves bounce off the walls of the microwave and into the food or other object being heated. These waves excite the molecules in the food, which causes them to move more, heating them up. The waves shoot through the food, allowing the inside to cook.
This method of cooking is in contrast to the more traditional convection oven. In convection ovens, heat is generated at a burner and passes into the air, and the hot air shares its heat with the food, gradually heating it. A larger oven requires more energy to heat up all the air, which is why smaller ovens such as toaster ovens preheat faster than larger ovens.
In microwaves, the air does not need heating. The air may become hot during cooking, but warming the food does not depend on this in the way that it does for convection ovens.