From the pattern:

Photos of vintage Scandinavian ski sweaters in Annemor Sundbo’s book Everyday Knitting (Torridal Tweed, 2001) were the starting point for the design of this old-style crewneck pullover. The body is worked in the round from the lower edge to the armholes, then the front and back are worked separately (back and forth in rows) to the shoulders. The sleeves are also knitted in the round to the armholes, then the caps are worked back and forth. To prevent the colorwork areas from drawing in, larger needles are used for the colorwork sections than for the solid-color sections.