Plain and simple: use.
You could try some Lexol on it, it works real good on tack too. Most feed & supply stores carry it. I use it on everything leather from boots to holsters.
chop a cord of wood with it on. moisture and friction breaks in leather.
A light coating of Ballistol (Yup - the, 'gun oil'.) prior to use will also do it for ya.
The ONLY way I know to "shrink" leather to fit your hand (or your feet) is to soak it in lukewarm water and then wear it until it is completely dry. Wet leather almost always shrinks. It can't shrink any smaller than your hand is large. It WILL conform to the size of the hand you put in it. I was once told by a VERY reputable cowboy boot dealer that the BEST way to get a perfectly fitting boot was to buy the size I could just barely get into. Then fill the empty boot "feet" with warm water and let them set until the leather had become completely saturated. Then put the boots back on and wear them until they are totally dry. They will have remolded themselves to the size and shape of your foot for the "perfect" fit. Since the boot dealers were expecting to do a LOT of business with me in the future, I don't think they would have told me this if it was not true. What do you think?