Because of disuse muscle atrophy. This happens in conservative management. Do not worry start exercising the muscles around injured area. Muscle bulk will come back by 2-3 weeks.

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Hi.., its not smaller , it must have become thinner.., this is called muscle atrophy.., occurs because the body part was not moving   so muscle bulk has been reduced. Don't worry when u do movements of your leg and weight bearing, it will become like earlier. Consult a physiotherapist for exercises direction. Tc

U need to learn a thera band exercises for ur ankle joint it is a kind of strengthening exercises for improving the muscle around the ankle u will regain ur strength and balance training exercises in a wobble board is a must followed by rigorous training like one leg standing with eye opening and eye closed for balances and co ordination