Why when the cast comes off your leg is often much thinner than it was?

Muscle atrophy: When a limb is immobilized in a cast, the muscles in that limb do not receive the same level of stimulation as they would if the limb were able to move freely. This can cause the muscles to atrophy, or shrink in size.

Loss of fluid: When a limb is immobilized, the fluid that normally circulates through the muscles and tissues can build up and cause swelling. Once the cast is removed, this fluid can drain away, leaving the limb looking thinner.

Compression: A cast can put pressure on the muscles and tissues in the limb, which can also contribute to muscle atrophy and fluid loss.

In most cases, the limb will gradually return to its normal size and strength over time with physical therapy and exercise. However, in some cases, muscle atrophy can be permanent.