well, its not just about yardage -- its about guage too.
(now these numbers are just a for-example, but you'll get the idea -- this is how I understand that it works -- but hopefully someone can confirm it!!).
Worsted Weight yarn is thicker than Sport Weight
* So if you knit 20 stitches by 30 rows of sport weight yarn on a size 7 needle -- it may measure 4" x 4"
* But if you knit the same 20 stitches by 30 rows of worsted weight yarn on a size 7 needle -- it may measure 5" x 5".
So even though there's more yardage on a sport weight hank -- you'll need more yardage to knit the same area as a worsted weight hank.