I just wanted to add my recent experience with barbour, and specifically the to ki to line. Just to preface, my jacket size is 40R and i wear a medium slim in button downs from jcew and a medium extra slim at brooks brothers. Waist is 32-33.
I bought the mountain parka in size L a few months ago and ended up returning it because it was too wide for my liking in the waist. Fast forward to a couple weeks ago, i got a good deal on the military jacket in sage,so i purchased it in size L also. It arrived and i was again a little disappointed with the fit. It fit pretty good in the chest, but it was boxy from the rib cage down. With the lining removed it was a little big in the chest, especially since i rarely wear more than 2 layers.
Frustrated, i called the Barbour store i where i bought the jacket and asked them to measure the shoulders of the medium they had in stock. The measurement they gave me was pretty close to the size large i had in my hand, so i bought the medium. The medium arrived today and i'm pretty pleased with the fit. Much slimmer in the body and still fine in the shoulders. Sleeves could be a tad longer, but i think i'm just used to having coats that are too big.
Long story short, if you usually wear a slim in shirts, etc. and don't have super gangly limbs, you may want to try your normal size in the to ki to line, at least in the jackets that tend to run a little bigger (those with zip-in liners?). I'm still waiting on the sports jacket i preordered in large, but i'm really interested to see if there are any notable differences in sizing within the to ki to line.