Everyone knows what it's like to do the laundry and then wind up having to re-break in a pair of pants, or squeeze into a shirt or skirt the first time after it comes out in the wash — but why does this happen, and can it be prevented? This video from DNews explains. For those who can't watch, the answer's pretty simple — natural fibres like cotton, wool and silk are, in their natural state, pretty curly and tangled. In order to be made into clothing, those fibres need to be stretched out, but given an opportunity to return to their natural, tangled state, they will. All of the mechanical energy that comes from washing, machining and drying clothes gives them the energy and the opportunity to slip those bonds and release the potential energy they get when they're drawn out and straightened to make the clothes that we wear. Some natural fibres do this more than others (wool being one of the biggest offenders), but they all do — and synthetics, for example, don't.
The reason for that is that they don't get the energy required (either through heat in the dryer or being bumped around in the washer) to deviate from their stretched and stitched forms. So as a solution, synthetic fabrics are often used to "fill the gaps" in clothing that's otherwise mostly natural fibres. That's why you see a lot of clothing that's a cotton/polyester blend, or cotton/rayon dress clothes, for example. The goal is to keep them from shrinking as much as possible when they're washed, and it usually works pretty well. Another solution is to spray down those natural fibres with anti-shrinking agents that keep the clothes from relaxing when they're washed.
You can see more about this whole process in the video above (and the link below), along with the three types of shrinkage involved with clothing (relaxing, felting and consolidation), when they're most likely to occur and which causes the most headache for clothes buyers like you and I.