Sizing is achieved by the use of two different weights of yarn, an aran/bulky weight for the medium & larger sizes, and a worsted weight crepe for the smallest size. Gauge is different for each size.

~ From Page 11 of the Knit One Below Book ~

Reversible Vests

Different gauge - different appearance. From a casual look with jeans to an elegant wrap in fancy yarn. The fronts overlap slightly and can be fastened or folded back to swing freely.

Although the vest has straight lines, the placement of your shawl pin will allow you to wear this garment in a flattering A-lined shape.
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Errata for only First Printing 2008
Page 12
Vitals column
Purple A yardage needs to go under the Bulky weight yarn icon and removed from the Medium weight yarn icon.
Vest
First sentence needs to end in slide (not turn).
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This pattern includes directions to knit the vest in two different yarn weights / gauges (Bulky / medium ).

Medium (Worsted) weight: 275 (325, 375, 425, 500, 550) yards of each yarn.
Bulky Weight: 225 (250, 300, 350, 400) yards of each yarn.