Organic fibers do not occur in a naturally straight form. They are usually too tangled to be used effectively. Manufacturers have to draw them out, stretch and spin them into threads before being able to make clothes out of them.
This pulling of fibers creates tension to be applied to the fabric at a molecular level. This tension is released by heat from the washer or dryer, which causes the fiber to revert to its natural size. As a result, natural fabrics that are made from plants or animals are highly susceptible to shrinkage, while synthetic fabrics do not shrink at all.
Please be aware that this phenomenon is as normal as it is natural, and appreciate the genuine quality of the fabric as well as its innate characteristics. We recommend going one to two sizes up from your normal size.