To supplement Ivwri's answer, here are asobu's thoughts on sizing. Very helpful starting place: Warning: Spoiler! (Click to show)
However, post-03/04 the pants are cut quite generously in the waist. It's not totally uncommon for pants in sz 3 to measure 18"+ in the waist - though there are exceptions of course - they are indeed meant to be bunched up with a belt or worn with suspenders, and they do fall correctly on the body. The specific pair (I think) you refer to definitely came in sz 2 and 4 as well, though it should be mentioned that those are some of the widest pants he's cut during the last few years and might not be the best place to start if you've never worn Yohji before. The typical YYPH cut is the wide pants with drawstring in the waist, those often come in the regular size set 2/3/4 and are huge in the waist. The ones on LN-CC this season measured 43" in the waist for a sz 2 (S).
About sizing in general:
The regular size run is 2, 3 and 4. This aw11 season they changed it up a bit and started selling 1 (XS) in the west, before those was usually only sold in japan in very small numbers. There are also sz 5 and 6 for the western market (not sold in Japan), those are also quite rare. A sz 6 is never ever a one size item though I've seen some sellers claim them to be.
A good way to approach Yohji sizing is to think about height rather than width, if you're tall you should probably size up one so the inseam is long enough, no matter how the waist is cut. This goes for jackets as well, you get the intended balance in length and sleeve length. It's a common "mistake" to size down because the measurements look big, remember that it's often better to size up than down when it comes to Yohji.
Also note that this refers to the mainline, the Y's pants are often cut considerably slimmer in the waist (with exceptions as always).
haha that's the one! sorry friend. I'm still waffling on whether to keep the jacket or not. I'll pm you if I decide to let it go