Clothing shrink happens when fibers absorb water/warm up and the edges of their structure (cellulose strands in cotton, protein scales in wool) stand up more and become more likely to catch on their neighbors. When they do, the fibers lose their ability to move independently from each other. As this happens, more and more fibers catch, turning the agitation in your washer and dryer into a sort of ratchet that shrinks the dimensions of the fabric.

Cotton can be un-shrunk with constant pressure. Ever had 100% cotton jeans tighten up in the wash that feel great after a day of wearing them? They shrunk and then your body undid the shrink. Wool isn’t as forgiving - the aforementioned scales clamp down on each other when they cool and dry and are effectively impossible to get free from each other. Wool can be treated before it is knit or woven into cloth to prevent shrinking and make it safe for the wash. Treated wool usually has a trade name attached to it like Superwash or something similar.

Both washer and dryer can cause shrinkage, but the dryer will cause more. Also natural fabrics will shrink more (by far) than polyesters. To your comment “I’m not gonna handwash. That’s just too much.” Well no one can blame you for that. But it’s still true that the gentler you wash your clothes, the less shrinkage you’ll get. It’s a balancing act, how much effort you’re willing to put in vs how long you want your clothes to last.

Personally I’m a natural fabric addict, there is very little non-natural fabric in my wardrobe. And I also do hardly any hand washing, but not zero. I often use a gentle cycle on my wash machine (top loader, sadly). Any non hand-wash garments that I still want to protect go in a garment bag on the gentle cycle, and do NOT go into the dryer. The bulk of my wardrobe gets dried but on low heat, and pulled before it’s quite all the way dry, because a lot of shrinkage happens as the garment goes from barely dry to fully dry. In particular my knits get the low temp dryer, and also any clothes which I consider semi-delicate, particularly well loved or barely big enough.

The combination of heat and agitation are needed to shrink fabric (especially wool). Heat makes the normally kinky, twisty, grippy fibers more flexible and looser. Agitation allows the loose fibers to reach other fibers farther away and grab onto them as they cool, pulling them closer, thus shrinking the fabric.

Heat and very careful stretching can somewhat restore the original size, but restoring the shape is harder.

• Hot water can shrink certain fabrics. Particularly cotton.
• Fabric softener can also shrink fabrics, though usually not enough that it no longer fits so much as returning stretched out shit back to their normal fit.

Wash with cold water and hang dry stuff as much as you can.