Heating things up makes them expand. When they cool back to their original temperature they (usually) return to (pretty much) their original size.
When things absorb water they increase in size. When they dry they return to (pretty much) their original size.
Why does washing some stuff on a really hot wash make it shrink?
It's the drying process that makes clothes shrink (apparently).
I'm sure there's some clever physicists on STW who could say why...
You'd be better off using fabric softner...
It's because the sort of wool mixes that most manufacturers use to make things like jumpers(cheap and recycled), and the thing that makes wool so warm is the same thing that makes it contract (and starts to turn in felt) when you wash it with too much heat and too much spinning. You can't do anything about it.
wool; buy the best quality you can, and read the care label!
You're using the wrong science - try biochemistry instead 
Wool is basically a protein which "cooks" (denatures) with heat. This causes the 3D structure to collapse, and your garment to shrink.
I read that it's like a sort of felting process. As you agitate the fibres they rub together and get more tangled up than they already are which packs them together more closely.
It doesn't have to be hot water, some wool stuff that Mrs Grips has woven shrinks a bit even on a cold wash. Hot water seems to make it worse though - Esme's idea seems raesonable to explain the role of heat 
The expanding when heating up is negligible compared to the felting effects mind.
As Emsz and Molgrips have already mentioned, another process which happens to woollen garments is "felting". This occurs because the wool fibre has overlapping scales from root to tip, so that the fibre can move in only one direction (like that cable-tie in your repair kit).
When a garment is subjected to mechanical action (such as spinning in a washing machine), the fibres move - but only in one direction. Thus the fibres become entangled and the garment shrinks.
But it's not only wool that shrinks is it? Cotton does a bit, doesn't it?
With cotton, it's usually the fault of the manufacturing process, which stretches the yarn. Washing relieves this "internal tension" and allows the fabric to relax, and therefore shrink.
Esme - Member
You're using the wrong science - try biochemistry instead
Ah, but it's all physics at the end of the day, isn't it?
I've got a pair of Dachstein style boiled/felted wool mitts. Apparently they start out 8 feet long. They are warm.
Maybe it should be a maths question?
Eeee, when I were a lass, it was called "Domestic Science"
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