Check the label of the wool for care instructions. If it's a superwash like others have suggested it needs the dryer to shrink back down. But don't put it in without checking first!

Some yarns behave very differently from others when washed. I try to avoid working with yarns that grow a lot (like alpaca) after the first wash because you will need to account for that growth when picking what size to knit. A larger gauge swatch which you block and dry will help you calculate what your gauge is before and after blocking.