I'm surprised it wasn't too small, rather than too large - that would be a more common experience, as the MontBell range in East Asia (Japan, Korea) is generally a size smaller than it is in the US. (Sometimes the labels show both sets of sizing).
Asia MontBell garment XL = US Montbell garment L
Asia MontBell garment L = US Montbell garment M
Asia MontBell garment M = US Montbell garment S
Maybe you were just unlucky, and got a mislabeled item.