I am 5'8" with a 36" chest who normally wears size 4-6 tops or Mediums. I bought 3 colors to try in size Medium (white, soft pink, and black) and ended up just keeping the Black supima tee, which I'm completely in love with. I have a wardrobe full of fun colors through Uniqlo's supima tees, and wanted to give Quince's new basic line a try. The neck line is much better made than Uniqlo's--it's wider and lays flat more easily, so there's less chance of wrinkling as you go.
However, the White supima tee is just too thin and sheer. I was wearing a ****-colored bra to try, and this would still only be something I would wear comfortably at home. The sheer-ness is very obvious in the neck area too, since the collar is thicker and sticks out as a more solid white color under the light, like you're wearing a hoop on your collarbone. The Soft Pink color is only a little better, but still more sheer than I like. However, I also noticed that in the two lighter colors run slightly small on me. I wear a 36" in the chest and usually wear sizes 4-6 in tops or just Mediums. The shoulder line is slightly too small for me, which makes the White and Soft Pink look rumpled, as the shoulder line doesn't lay flat across when I shift. I have never had a problem with Medium shirts not fitting in the shoulder before, and usually have to size down to Small, if anything (I wear a women's Small in the Uniqlo supima tees).
The Black tee I love and will keep, because the shoulder line/sizing issue is not as obvious due to the dark color. I really enjoy how soft the material is; the Black tee is probably the most comfortable shirt I own at this point. If Quince can fix the sheer material issue (Uniqlo's White is almost twice as solid-looking), then I will absolutely be back to pick up the rest (possibly in a size Large).