origin

Well, I'm originally from a city called Utsunomiya.

The origin is the beginning of something or where something started. Originally is the adverb. Notice the following:

1. The origin of basketball is America.
   
2. I now live in London, but I am originally from Liverpool.
   

ever since

And I've been there ever since.

We use the phrase 'ever since' to talk about a point in time in the past when something began that continues into the present time. Notice the following:

1. Ever since I had a baby, I sleep a lot less.
   
2. I moved here for college and have lived here ever since.
   

in terms of

It's very small in terms of the size, you know, of the area.

Here, the phrase 'in terms of' is used to show specifically what we are talking about. Here Hitoshi isn't talking about the the size of the people being small, he is talking about the size of the country compared to other countries in the world. Notice the following:

1. In terms of housing, New York is one of the most expensive cities in the world.
   
2. In terms of sports, my high school is very strong. Not so much in music though.
   

two-thirds

Two-thirds of the area is covered by forests.

Two-thirds is written form of the fraction for 2/3. It means that if you divide the land in Japan into three parts then two of these parts are covered by forests. Notice the following:

1. Two-thirds of the students failed the test.
   
2. We are two-thirds of the way to our destination.
   

leader

And who is the leader of the country?

The leader is the person in a group with the most power or responsibility. The other people in the group follow the leader. Notice the following:

1. Most people feel the President is poor leader.
   
2. I would not want to be the leader of a company as it is too stressful.