The calories from sushi come from the rice. To put it in perspective, one piece of nigiri (the little mound of rice with a slice of salmon on top) is are anywhere between 50 - 70 calories. I could easily eat 10 of those tasty little morsels and there's at least 500 calories!!
Sushi hand rolls are about 150 calories each depending on the size so that's not tooooo bad!