Hope this helps… Jeans were originally sold “Shrink to Fit.” I think because they were only available in 3 sizes. The wearer would put them on and go into a pool of water and then allow the jeans to dry on them. The result was a snug personal fit. Shrink to Fit jeans generally shrink about 10%.
Most jeans nowadays have already been Pre-Shrunk and washed multiple times (at least 12 times or so). Pre-Shrunk jeans will not shrink very much, and will always return to their default size about some time of wearing.
There are some brands of jeans that can still be purchased Shrink to Fit. The most well known, and my personal favorite are Levi’s 501s Shrink to Fit (These are the original jeans). You can find them almost anywhere Levi’s are sold. The tag on the back pocket will indicate that they are Shrink to Fit. The denim will be dark indigo and very stiff and rough. It is reccomended that you choose a pair 1″ larger on the waist and 3″ longer on the inseam.
The jeans can be shrunk a number of ways. You do not have to be wearing them to do this. I just fill up the tub with the hottest water possible and let the jeans sit in there for about 30 minutes. I take them out and hang dry them over night. Then wash them and hang dry them again, and finally wear them for a long time without washing them to break them in.
If you want to purchase Shrink to Fit jeans go to Levi.com or visit your local Levi Store or Outlet.