generally chinese and japanese people do not eat food prepared the same way as they do for us gluttonous Americans, nor do they eat anywhere near the same quantity.
You will be very surprised how many calories your downing with a simple spicy tuna roll. Add some tempura, and chicken teriyaki, some noodles, and you will be eating 2000+ calories in single meal. Its like going to any restaurant, french, italian, chinese, if you dont watch what your eating, you can take in alot of bad calories.
That being said, I enjoy sushi at an amazing restaurant nearby 1-2 times a week. I do watch what I eat because I know how easy it is to eat poorly and give in to just make my taste buds feel better, and me feel worse 20 minutes later. If you want to stay healthy, try not to get, or limit anything with tempura (fried batter), unagi (broiled eel), any roll that is "spicy" or has "crunch" in it. I love spicy anything rolls, but they are generally packed with extra sugar/oil to get that flavor. I am a huge unagi fan (its my favorite thing), but even I need to limit myself as its very fattening (the skin is).
If you want to eat something healthy, I usually get seared tuna salad with avocado as a starter, and dont put any ginger sauce on it. Get some edame, some green tea too. Also, any type of sashimi is generally very good (pieces of fish without rice) even salmon which has healthy fats. Some are more fatty than others, but any fish is very healthy, except maybe toro which is very fatty or eel. Order simple rolls without alot of craziness to them, or eat just a few pieces of specialty house rolls. Again, just keep in mind what you are eating. Everything in a japanese restaurant will taste good, and you will want more of it, so keep things in moderation because its easy to splurge.