I guess the perception of sizing is different for different people, but I wouldn't say this top runs small. I have narrow shoulders and a slender build at 5'6'' 125 lbs, and I usually buy small or x-small tops. Usually x-small is totally form-fitting for me, whereas small gives a little bit of room around my ribs and stomach. I bought this top in a small, and I think it's true to size. I can wear a layer under it. I would probably reorder for an x-small if it were primarily for fashion rather than function. Granted, I like my tops close-fitting, but this one still has some wiggle room for me, particularly in the arms and at the bust.

I jumped straight to commenting about sizing because really, you should just buy this top. It's thin and light but is an amazing body temp regulator. I wore it hiking with a camisole underneath and the temperature swung from 50F (and windy) to 70F (and sunny). I was chilly on the low end until I layered up and a bit too warm on the high end, but I never experienced the oh-my-god-I'm-freezing or the oh-my-god-I-have-to-take-this-layer-off-now moment. It keeps me just below the overheating point where I have to take a layer off, which is important when temperature flux is likely.

I fell in a blackberry bramble wearing this top (now I really sound like a knowledgeable and skilled outdoorswoman, right?), and expected to find some serious damage from all the snagging, but the top made it out in better shape than I did. It looks brand new.

By the way, I also bought the BodyFit+ 200 Scroll Print V-Neck Top version of this, and I'm not in love with it. I assumed it would be just the same as this top aside from decoration, but it doesn't look as nice. Could be the fact that it's mostly solid colored rather than patterned, and ivory rather than darkish blue tones, but it seems to fit differently. And the ivory color is sheer. I mean see-through. I mean you can see every freckle underneath. Just a warning. Buy this one instead!

So far I'm really happy with my jacket. I haven't had any really heavy rain to try it out in so far, but in moderate rain it has been great.

Pros:

-Great hood. It's huge without any cinching (haven't tried it, but I bet you could wear a helmet underneath) and you can both tighten it around your face and tighten a cord in the back so it pulls back from your face. With this adjustment my face doesn't get rained on, but my hood also doesn't hang in front of my eyes and obstruct my vision.
-Nice features for being as light as it is. Pit zips, double flap over front zipper, lots of adjustors.
-Pretty good for avoiding clamminess.

Cons:

-Kinda short. The proportions in the photo are very misleading... the width has been pulled in on the sides which makes it appear longer. It hits me at the top of the hips and exposes my stomach when I raise my arms above my head. I definitely wish I had bought the longer parka version.
-The fabric around the cuffs is super stiff. The first time I wore it with the wrists tightened, it chafed my wrists big time. After breaking the fabric in a little it's better, but still not highly comfy.

As for fitting, I'm 5'6'' 125lbs with a small torso, and I bought an extra small. I can layer and layer and layer underneath, I can even fit my down jacket under it. I usually buy a small or extra small in jackets. Unless you don't mind a baggy jacket, size down.