I think Ibex runs big, in general and their sleeves are always quite long (which I know is a godsend for many!).

Icebreaker generally runs slightly smaller than Ibex in my experience (and tops are usually very long in the body).

I have also gotten specific items from both makers that buck this trend completely AND items that become totally distorted after one washing (clearly due to some defect). It can really be a crap shoot!

Smartwool seems to run smaller but I have limited Smartwool experience unless you count socks (which I think run big!).

Trisk - I don't know what size my Pez vest is (I'm at work, it's at home!), but I can check for you tonight. I do know that my vest fits looser than my Shak's, so if they are the same size, then it could be that the Pez runs larger. That's my guess, but I can't say for sure without checking (plus, vest sizing could be different than sweater sizing, of course)... I'm not much help, am I?