Why do slippers always stretch?

I bought the most wonderful comfy pair of slippers last Tuesday - not the most beautiful I'll admit but so warm - and they were snug.  I thought that they'd stretch slightly as slippers always seem to but that wearing them with socks  would be ok.   One week later they are like small boats slopping around my feet....John says if they get much bigger he'll wear them and I can buy another - and much smaller - pair.

So why does this happen??  I realise they are made of fabric which has some looseness, but so do my clothes and they don't go saggy and baggy.  All slippers that I've bought seem to do this, I'm not getting at this particular make of slipper.  I've had well-known high street brands in the past, locally made uk ones and  cheap and cheerful from the local market.  Some wear better than others and the price doesn't seem to have any relationship to the sagginess or bagginess or even the quality of the finished slipper.

Do I buy cheap and cheerful and frequently?  Do I buy something more expensive and then mutter and moan when  they don't last longer?  Do I give up on slippers altogether and wear sox indoors (but then they don't keep my feet warm....  )          I have some very warm and  toasty slipper- socks  that do what they are supposed to:    i.e.  warmth and slippers all in one.......but they are hard to find and I have been lucky enough to have been sent them as a pressie.

When I find the perfect slipper which I'm sure must be out there somewhere, probably at some astronomical price - or maybe not in these days of financial doom and gloom - then I'm sure I'll be ready to give them a 'road test' and report back.  Any slipper makers out there reading this - please take note!