to put it that way

Norway is not as crowded as Tokyo to put it that way.

We use 'put something that way' is to explain it in a way that they person you are talking to will understand better. In the case of the example the person you are speaking to is familiar with Tokyo, but not Norway. Notice the following:

1. Let me put it this way, it's easier than climbing a mountain.
   
2. Well, when you put it that way I can understand your point.
   

three times

Tokyo's three times the population of Norway.

If the population of one city is "three times" bigger than another city then it has 300% more people.  For example, if Norway had a population of 1 then Tokyo would have a population of 3 because 1 x 3 = 3.  Notice the following:

1. Today feels three times longer because I am sick.
   
2. His apartment is three times bigger than mine.
   

clean air

The best thing about Norway is the clean air.

"Clean air" is air that does not have contamination.  Notice the following:

1. We prefer to do exercise in a place with clean air.
   
2. The leave the city every weekend to enjoy the clean air of the country.
   

cross-country skiing

I used to do a lot of cross country skiing.

"Cross-country skiing" is skiing on flat land instead of down a hill.  You use your body strength to push you forward.  Notice the following:

1. There is an excellent cross country skiing trail through the woods.
   
2. We went cross-country skiing after the first snowstorm.
   

physical

Downhill skiing is a bit easier, less physical.

When an activity requires a lot of movement and strength it is a physical activity.  Downhill skiing is less physical because you don't have to move your body a lot.  Notice the following:

1. I didn't know that swimming was really physical.
   
2. He was very tired after a day of physical work around the house.