If knuckle cracking contributes to fat bones...if i quit will they get thin again? cos my knuckle bones are all fat now after a lifetime of cracking.

Not likely, but: Typically one bone growth has occured, it does not regress--maybe thinning out, but not reducing size. So, if excessive cracking has resulted larger-than-usual knuckles, stopping it wont make them smaller. It will stop getting larger. Fortunately, knuckle-cracking is not associated with joint problems. If "fat-bone" u have is excessive, there maybe other problem. Consult doc. Good luck.