The yarn, a DK weight, is knitted tightly (7-8 stitches per inch) to produce a warm, cuddly mitten.

The different sizes are achieved by changing the gauge.  The charts and instructions are the same for all sizes. 
 
Small: gauge:  8 stitches/inch, circumference of cuff:  7.5 inches
Medium: gauge:  7.5 stitches/inch, circumference of cuff:  8 inches
Large: gauge:  7 stitches/inch, circumference of cuff:  8.5 inches