The key is the way you ride a bike. If you use a large transmission ratio, that is, a large toothed plate and a small flywheel, your legs will become thicker. Because riding resistance is great, just like bodybuilders have to lift heavy barbells to exercise their muscles.

On the contrary, with a small tooth plate and a large flywheel, people will feel effortless and their legs will become thinner. This is my personal experience. I've been practicing driving for eight years, and my legs are still thin (thinner than women's). My bikers nicknamed me "the king of beautiful legs".

As for the problem of "ordinary professional cyclists have thick legs", their purpose is to ride faster, at the same time improve their cycling frequency and strength, and to ride faster than others, ride with a larger transmission ratio. This is not the same as the purpose of ordinary people.