Today I came across these big fellows.
This is a giant flower mushroom (大花菇 dà huā gū) - a shiitake considerably larger those normally available.
Interestingly, they were also considerably cheaper than regular "flower" mushrooms". At ¥60 per 500g as opposed to around ¥100 per 500g for the regular sort. Also, these were supposedly imported from Japan, so the price difference seems even greater.
It seems the little ones are more highly prized.