Why sushi is more popular:

"Dish" can mean food or plate. Which do you mean? 

Here are 2 ideas. The first idea talks about food. The second idea talks about plates.

1)Delicacies characterize Japanese food. They are served in small amounts, along with other exquisite dishes, and often in an enjoyable environment.

2)Delicacies characterize Japanese food. They are served in small amounts, upon exquisite dishes, and often in an enjoyable environment.

I do not understand why you use "species". Do you mean "spice" or "culture"?

Here are 2 ideas:

1) While Chinese food highlights it's culture's richness. The two countries have their own food style.

2) While Chinese food highlights it's spices, richness, and good favour. The two countries have their own food style.

Here are 2 ideas:

1) One day my friend mentioned that sushi is more popular than any Chinese food, which pushes me to seek the reason. Sushi, as the representation of Japanese food, perfectly combines the meal and garnish, providing an enjoyable taste for all kinds of people.

2) One day my friend mentioned that sushi is more popular than Chinese food, which pushes me to seek the reason. Sushi, as the representation of Japanese food, perfectly combines ingredients, providing an enjoyable taste for all kinds of people.

Here are 2 ideas. The second idea sounds more natural:

1)The small size and curved shape make sushi very convenient to swallow. And, you don't need to share it with others as Chinese food requires you to do. Last but not least, It is rich in nutrition. The above advantages may contribute to sushi's popularity.

2)The small size and round shape make sushi convenient to eat. And, you don't need to share it with others as Chinese food requires you to do. Last but not least, sushi is highly nutritious. The above advantages may contribute to its popularity.

I hope that is helpful :) You're English is very good!