Petite sirah is so named because it has less tannin than syrah.
Petite sirah, also known as durif, has an enormous amount of tannin. The variety is a cross between two French grapes—syrah and peloursin. Petite Sirah was crossed in the 1860s, by a scientist named Durif and is sometimes spelled petite syrah. Petite sirah grapes are generally smaller than syrah grapes (hence the “petite” in the name). As with all small grapes, petite sirah has a high ratio of skin to juice. Since tannin comes primarily from a grape’s skin, small grapes like petite sirah often have a considerable amount of tannin. In the end, there is nothing petite about petite sirah. A similar example is petit verdot (petit in this case is spelled without the “e” since the word verdot is masculine). There’s nothing petit about petit verdot either.