A US size 6 is a UK size 10
A US size 4 is a UK size 8...
A US size 0 is a UK size 4.
Clothing sizes have gradually got bigger and bigger, but some small, very slim people have stayed the same size.
A US size 6/UK size 10 now has bigger measurements than it did, say 20 years ago. So - what are the people that were a size 6/10 meant to do? Drop down a size - to 4/8, that's what.
Sizes 4/8, 2/6 and 0/4 were virtually unheard of 20+ years ago, as the existing sizes were actually small enough to fit very thin, petite people.
But as the measurements gave increased, they've had to add in 'smaller' sizes to accommodate all the same people who naturally haven't increased girth in line with the clothes sizes.
I mean, come on. 
Human beings as a whole are most definitely not getting skinnier and skinnier and smaller and shorter. In this time of plenty, of processed foods, of huge portions, of fat and sugar and everything we could possibly want PLUS less daily movement and exercise - you cannot possibly believe that humans are evolving into slimmer, leaner versions of our historic and pre-historic selves. It just doesn't make sense.
No, we are getting bigger. Our clothing sizes are getting bigger to reflect this.
They've had to add in smaller sizes (2/6 and 0/4) to accommodate the relatively small number of people whose girth has not increased, and who used to buy clothing sizes 4/8 and 6/10).