It really started with Jane croft a couple of years ago, although small pigs have been a problem with the sanctuaries in the US for many years, its a spin off from pot bellies I think. Originally it was runt to runt, but then people like rob rose who is the biggest problem after Jane croft, started putting large traditional breeds to kune kunes and pot bellies to get the colours and of course you couldn't guarantee the size. Jane croft was buying up litters everywhere regardless of what they were or their history, so the spread of micro turned large pig spread. JC is very good at marketing herself, so the micro trade took off. It pandered to the plebs who wanted the next new fashionable thing.