WOOL SHRINKAGE IS AFFECTED BY MANY FACTORS...
MAYBE IT'S TOO TECHNICAL, BUT THE TYPE OF WOOL YOUR GARMENT IS MADE OF DEFINES ITS SHRINKING RATE
The type of wool changes how much it will shrink. The longer and smoother the wool fiber is, the more heat and agitation it takes to felt and shrink. Merino wool, for example, has fairly long fibers without much natural crimp. So it takes longer to shrink than shetland wool, which has more crimp and shorter fibers. The more a wool fiber naturally curls in on itself, the more it will shrink up. Unfortunately, most of the time, you don't know exaclty what kind of wool was used to make your garment.
THE WAY YOU WASH YOUR WOOL GARMENT CAN LITERALLY MARK ITS DESTINY...
Water is not the main main problem. That's because wool is made up of a series of interlocking fibers that are held together by scales. These scales help keep moisture away from the wool fiber, which is why wool naturally repels water. BUT If you put wool in hot water, the heat will open up the scales and allow water to penetrate into the fiber. Wool fiber swells when it becomes wet and then contracts when it dries. This is what causes wool to shrink when you wash it in hot water. So If you don’t want it to shrink, just avoid washing it in really hot water. Handwashing in cool or slightly warm water is always recommended for wool garments.
...NOT TO MENTION THE WAY YOU DRY IT!
Your woolen garment will shrink in the dryer, because it makes it subject heat and friction. Even when you use the low-heat level, the machine does use friction to dry your wool, which can cause pilling and felting. Airdrying wool garments is much better for them, since it prevents shrinking. After washing your wool garment, just make sure that your garment is not dripping wet by gently squeezing the fabric, and then lay it flat on a towel to air dry.
CHOOSE WISELY, THE DETERGENT YOUR USE CAN PLAY A KEY ROLE IN AVOIDING OR GETTING WOOL SHRINKAGE
An enzyme-free detergent is the only kind of cleanser that is safe for wool. If the cleanser you are using contains enzymes, they will damage the proteins that make up the wool fiber. The more damage to the proteins, the more easily the scales interlock and shrink the fiber.