If you put a sweater in the washing machine, it may shrink. So why doesn’t fleece, which is also basically wool, not shrink when it rains? The main cause is friction.
Like the fur of all mammals, the wool is longitudinally covered with numerous overlapping small scales. The scales cause the yarn to slide in one direction more easily than the other. This also explains that if you stroke a hair with your hand, you will find it easier to swipe from the bottom to the tip of the hair than the other way around.
When sheep stand in the rain, the hairs in their thick coats swell and the scales soften.
When sweaters are put in the washing machine and kneaded, this one-way flake becomes a problem. When the wool fibers rub against each other, the scales are like little hooks that only allow the yarn to move in one direction.
Water makes things worse. The water swells the wool, brings them closer together, and softens the scales just enough to make them easier to stick together but not soft enough to slip through.
The heat also helps the wool fibers to stick together by becoming more malleable and exposed, just as cooked spaghetti strands collide more than hard, raw fibers.
During the washing and drying process, millions of tiny hooks on the thousands of yarns of the sweater cause the wool to become increasingly tight and the shirt to shrink.
When sheep stand in the rain, the hairs in their thick coats swell and the scales soften. But the fleece is not so kneaded that the fibers get tangled together and the coat shrinks.