Doh, I just remembered, it was the midweight LS crewneck that I tried on, not the zip-T. (REI had both the midweight crewneck and the midweight zip-T in stock).

Gah, I'm getting senile.

So allow me to rephrase ... in my experience, I found that the smartwool LS crewneck in the midweight wool is much more form fitting (as in, purple sausage casing) than the smartwool LS crewneck in the microweight wool.