The joints cannot become thinner, and the bones that have grown cannot be eliminated. If you are born with a guitar, you can only rely on more practice. If it is because you like to break hands, then quit as soon as possible. , the opponent will be very bad in the long run. If you have to, you can have surgery, but it's very dangerous and the hand may not be able to return to its former flexibility.

There is no way to make it thinner

But you can make other places a little fatter

It will look more proportional