Kevin Sparkswrote a review of Arc'teryx Atom LT Insulated Vest - Men's on August 30, 2016
Familiarity: I've used it once or twice and have initial impressions
Fit: True to size
This is a nice vest. Arcteryx makes quality products. My main use will probably be casual but I wouldn't hesitate to take it hiking in the winter. After reading the reviews, I decided to go with a Medium. For my size, the Medium fits close to the body. I can wear it over a t-shirt/ flannel shirt combo and I can also wear it over a light synthetic base layer/Arcteryx fleece jacket combo (the jacket is similar to the Arcteryx Delta LT Fleece Jacket - mine is an older version but looks to be about the same weight of fleece). With these combos, I have zero extra room in the vest. The fit in the chest feels the closest but it's not pulling the stretch material on the sides of the vest. The waist is spot on. I can run a single finger all the way around the inside hem but that's it. There's no drawcord at the waist so I wanted the fit to be close.
Just to be safe, I decided to contact Arcteryx and see how the size Large would compare to the Medium. Here's what I received:
Here are the measurements for the Atom LT vest.
Chest Width 113cm
Waist Width 107cm
Bottom Width 107cm
FT Length 69.6cm
CB Length 74cm
Chest Width 121cm
Waist Width 115cm
Bottom Width 115cm
FT Length 72.4cm
CB Length 76.5cm
So, essentially, you're picking up a little over 3 inches in the body and about an extra inch in the length.
For me, I decided to stick with the Medium. I like the close fit at the hem and I would rather have the vest be a little snug vs. the extra room the Large would provide. I'm ok with the length too but, if your stretched out (say, on a bike), the extra inch of length, on the Large, might be nice. If you're my size, it seems like you could wear either size depending on your preference for fit and desired use.
By the way, my jacket size is 42L and my waist is 34 in most pants/shorts. Hope this helps.