I have some merino sweaters, tshirts, gloves, socks, long johns, boxers, hats.... from different brands and none have shrinked more than one size down. I even have tried to make one shrink 2 size down and couldn't. I washed and dried it at max temp several times, ironed, cooked in the microwave, boiled it... (it was probably preshrunked) (if someone knows how please tell me, i still haven't found a way
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I have one brand that shrink more easily than others though.
Now that i think about it i have a european brand that shrink like crazy but it's angora wool and not merino. I've also shrinked a few of my lambwool gears in the dryer to make them fit just the way i wanted.
But with my merinos it's pretty hard to get one size down.
You probably got one of bad quality.