Thicker yarn takes more.
Measure the number of sts you knit per inch in center row of sample. Measure rows per inch, too. Measure the hand of the person for whom mittens are to be made, measuring around joints below the fingers, same as measure for glove size. This measurement multiplied by the number of sts you knit per inch equals the number of sts you need to cast on for mittens. Using needles used to get your gauge, cast the required number of sts on 3 needles.