The time it takes to heat up two microwaves of different sizes at the same time can vary depending on several factors. The primary factor is the wattage or power rating of each microwave. Generally, microwaves with higher wattages can heat food faster than those with lower wattages.
Other factors that can influence the heating time include the starting temperature of the food, the quantity and density of the food being heated, and the efficiency and design of each microwave.
Assuming the two microwaves are operating at their maximum power output, and all other factors being equal, the larger microwave will likely take a bit longer to heat the food compared to the smaller one, as it needs to distribute the microwave energy over a larger interior space.
To get a more precise estimate, you would need to know the wattage ratings of both microwaves and the specific food item you want to heat. Generally, you can refer to the microwave's user manual or check the label on the back or side of the microwave for the wattage information. Once you have this information, you can then calculate the estimated heating time based on the specific wattage and the guidelines provided in the microwave's manual.
Remember that it's essential to follow the manufacturer's instructions for heating food in the microwave to ensure safe and even cooking.