Searched until my eyes fell out. (again)
A link to a previous answer is as acceptable as an answer in this thread.
I remember the shrink to fit jeans we wore in the 70s. Some probably still buy them… Some of my friends would wash them and wear them wet. The jeans were huge before washing. Shrunk in the wash. But, did they also shrink while drying?
And how about stuff that isn’t shrink to fit but still ends up smaller after laundry day? What happened there?
Cotton shrinks when heated. I think it may shrink some if it gets wet and then just dries out. Putting it wet in a hot dryer is the double whammy. I’ll bet they didn’t shrink at all in the washing machine.
Calling this “shrink to fit” is nothing short of marketing genius. It’s making it so it won’t shrink that’s hard. Almost all natural fibers will shrink somewhat when washed and dried, wool comically so.
Are you sure that it isn’t my problem, expansion of the contents within? 
I don’t have a specific answer regarding shrink-to-fit jeans, as they seem to shrink unusually much.
Anyhow:
Most fabric will shrink from a little to very much depending on the handling, i.e. washing, spinning, tumbledrying, etc. This handling (and heat) will cause the fibers to change size/shape, move a bit, etc.
One of the larger factors regarding shrinking fabric (esp. cotton) is in the manufacturing. When a fabric has been woven, it’s then bleached/dyed, washed and dryed. And a part of the drying process is that the fabric is stretched a bit (mainly across the width), so that the width will be even on the whole roll. Cheap fabrics are usually stretched a bit more than they should be, wich cause them to shrink considerably when washed.
Careful ironing with lots of steam or a wet tea-towel, can stretch the fabric yet again, if it shrank too much in washing (I don’t recommend this for shrink-to-fit jeans though )
Wool is a little different. It’s hair and hair has tiny scales. When the wool is wet (and warm) the scales will rise a bit, and when the wool is HANDLED, then they will rub against each other and cause the fibres to felt - and ultimately the garment will shrink. It’s the wetness, warmness and handling of the fibres/fabric put together that creates the felting. When wool is dyed, the dying solution is usually very warm (close to boiling point) - the trick is then to handle the fabric/yarn as little as possible.
It’s definately the dryer that causes the shrinkage around my house. If I let them drip dry, my jeans are at least a half size, if not a whole size bigger.
NoClue…I thought you had a NoClueGirl who took care of all this for you…?
…and we probably should give some credence to Billdo’s expansion theory…
Is the shrinking of cotton irreversible?
In the almost year I’ve been here, I’ve gone through two. And now I’m wondering if i should date the stripper or the new girl.
It’s gotta be my laundry skills. Hence, the question. 
I know I’ve outgrown some of my older smaller jeans. But this happens alot when buying NEW clothes, too. Perfect fit in the store, tight the next week. I know I’m not retaining water…
How does dry cleaning affect my wool suits and pants? They seem to be no smaller. Even the ones that are several years old. Or is it because they are cut for a looser fit to start with?
Which one has the best laundry skills? Maybe a laundry pop quiz is in order…