I bought a Adventure Tech Gortex ECWCS reversible parka off ebay in a size Large Regular.  I am about 5'11" 200lbs. - a bit heavy for my height, size 36 waist.  The parka fits fine, but then I needed to get the pants.  I finally found the reversible trousers in Large Regular.  When I got them, they hardly fit over my normal clothes, which as I understand it, is the purpose of the ECWCS.

The pants cause my pants underneath to bunch up and rise and then I can't crouch or squat or hardly sit down.  I then found a pair of non reversible Gen II Adventure Tech trousers in XL regular.  These things are too damn big.  The got me over the last hump, but I look like a rapper and I have to basically tie them around my waste.  Now I have two brand new pairs of Adventure Tech Gortex pants, and they won't fit me right.

What am I doing wrong here.  

Nothing really.  You are at a bad size for many Army clothes.  Too big for large, but not big enough for XL.
The same thing often happens between med and large.

You can pick which of the two sizes works best for you, or pig out and get fat, or lose a few inches.