I think there was some artistic license for the images of very fitted pants, but they were actually more fitted than modern men's straight-leg pants. The same style pants were portrayed in southern Europe, Thrace, and Phrygia, but I think the Alexander Sarcophagus gets it right with the realistic images of the Persians. Note that the guy in the foreground has the same zig-zags as seen in the other regions.
Some other pics are here:http://www.mlahanas.de/Greeks/Arts/Alex ... hagus.html
(As a side note, this guy has seperate soles on his shoes.)
It looks like the pants either have attached feet or straps like stirrup pants to hold them in place inside the shoes. The zig-zags, the hatching on the Gundestrup cauldron, and the diamond weave of the Thorsberg trousers lead me to believe that most pants were made with herringbone cloth to give more stretch. Cutting your blanket on the bias (diagonally) gives the same effect. Unfortunately, we don't get to see the pelvic area. They could very well fit like diapers, and you just hide it under your tunic. I dunno.
This guy looks a bit baggy around the pelvis, with the top of his pants turned over, maybe a belt underneath? A bronze statuette from Alesia, France (Aw, man, that's huge. How do you make things smaller?):
In reading about trade, I've always gotten the impression that no matter the region, the trade was typically either fresh fabric or elaborately decorated torso-covering items that wouldn't need to be so fitted. I could be totally wrong about that, but that's just what it has seemed like. The fresh fabric is easy to transport, and the elaborate pieces have their value in the decoration.