Why do size smalls shrink, but mediums stretch out?

[tiapam]

I like knit tops. I am usually a size small but I prefer to wear my clothes comfy and loose. So I buy both smalls and mediums, depending on what feels okay and still looks nice. And the smalls *always* contract and get smaller and the mediums *always* stretch and get bigger. Why can't the small stretch out and the medium shrink a little???

I can't win.

[sunshine873]

I would have the exact same complaint, but for me it's Medium & Large! Ugh.