Depends on what you have on/in the sushi. Typical Japanese sushi is not fattening as they use raw fish and vegetables, and they eat in moderation. However, if all you like are those westernized sushi stuffed with cream cheese, fried food, etc. then there's a problem.
Sushi is very healthy. There are carbs in the rice, but there is so little rice, it shouldn't really matter. Just think of it this way, how many truly overweight Japanese people do you generally see? Sumo wrestlers aside. lol! Enjoy. I like the Spider Roll myself. Very fattening: fried crab.