No, running a microwave with a smaller than intended turntable will not increase magnetron reflection. The purpose of the turntable in a microwave is to ensure even cooking and heating of the food by rotating it and distributing the microwaves more uniformly.
The magnetron is the component in a microwave oven that generates the microwaves. It emits these microwaves into the cooking chamber, and they bounce around, interacting with the food, until they are absorbed and converted into heat. The design of the microwave's cooking chamber is specifically engineered to prevent significant amounts of microwaves from reflecting back to the magnetron.
The turntable's primary function is to rotate the food, so all parts of it experience similar exposure to the microwaves. Without the turntable or with a smaller one, certain areas of the food may receive more microwaves, causing uneven cooking or heating.
While a smaller turntable may affect cooking performance, it does not directly impact magnetron reflection. The microwave's design and safety features are meant to minimize reflection and ensure that the microwaves are directed towards the food rather than being wasted or reflected back to the magnetron.