When grinding with a centerless cylindrical grinder, why is the same grinding wheel less and less, and why should the grinding wheel be repaired frequently?

For example, a big cake needs 10 in the outer ring, 8 in the inner ring and 6 in the inner ring. This is because the radius is smaller and the circumference is smaller, and it is impossible to grind 50 wheels at a time like a new grinding wheel forever. I don't know if I know.