I'm a US size 0 or 2 as well and have trouble fitting into EU 4 sometimes.  I've been extra shitlorded I guess by a Russian and an Italian now for even asking for an EU S top.  I'm 5'6" and 120lbs and a pear shape so my upper body is pretty small but there's no way to zip up a real S jacket.  I think I'm what average should be.