I just got my jacket in the mail and tried it on, and I was extremely nervous about the fit. My first Columbia fleece jacket was a medium and too big, so I got a small and that fit perfectly. Then I got a durable waterproof jacket in a small, but if it was any smaller I wouldn't be able to move. So I weighed my jacket options and decided to go for a the Switchback II in a medium.
I'm glad I didn't get a small- the medium is pretty perfect. I am a size medium, 8/10, and I have a smaller waist in comparison to my hips, so sometimes I can get away with something in a small size but in this case I'm glad I got the medium because it fits at my ribs just right and still has room to wear a sweatshirt or thick sweater underneath. (which I would recommend doing because this jacket is very lightweight and isn't meant for warmth.)

So if you're someone with a smaller waist and bigger hips like me, I definitely recommend this jacket. I also like the tall stand-up collar, and if you don't like that, just take out the hidden hood!