A motorcycle jacket should fit well for maximum effectiveness. Unfortunately, not every individual is sized exactly to the sizes on the market, which means a jacket can be too large when you buy it. Although you can expect some shrinking over time, typically it isn't enough for a slightly large jacket to fit a smaller body. The materials make a difference in the most effective shrinking methods, but you can shrink most motorcycle jackets.
Dry clean the jacket. Dry cleaning chemicals will shrink some materials, like leather, small amounts. Taking the jacket to a dry cleaner or using a home dry cleaning kit and a dryer might shrink the jacket enough that further shrinking is unnecessary. That will depend on how much you need to shrink the jacket.
Wet your jacket with warm water. Allow it to become as wet as possible, then dry in a dryer at high heat. This will shrink most motorcycle jacket materials, from denim to leather. Check the jacket every few minutes so that it doesn't shrink too much.
Try the jacket on to determine if the size is right. If it is still too large, repeat the process with water and the dryer. Keep in mind that motorcycle jackets are designed to be worn over sweaters or other bulky clothing. Be sure it's large enough to wear a sweater underneath.
Keep in mind that some materials, like leather, will naturally shrink slightly during normal wear. Conditions like rain, humidity and cold weather will make it shrink. Avoid shrinking a leather jacket to an exact fit. If the jacket is too large for a comfortable fit even after shrinking, you can take out the hems and sew them slightly smaller. A tailor can resize the hems or you can do it at home with a needle and thread or a sewing machine. do this only if you can't get the jacket to shrink enough for a proper fit.