As far as I know their sizing is a little snug. In most clothing I'm a standard S, but I'm broad over the shoulder, upper back and upper arm, so I sometimes have trouble getting closely tailored tops to fit. I bought the Ever dress in both sleeveless and longsleeved - I could squeeze into the S, wool is forgiving, but fit much better into the M.
As maybe shows on the photo it's close but not stuck to me around the shoulder and upper torso, then flares gently out from the base of my ribs. If you're petite in the upper body you're more likely to fit the standard size.