As I understand 都市 [toshi] means [city].  
And therefore it could be any city, which is not town.

In Russia town means over 12K population.  
100-250K pouplation = big town.  
250-500K pupulation = small city.  
500K-1M population = city.  
> 1M population = large city = megapolis.

There is Japanese word 大都市 [daitoshi] which means large city.

But for some reason there is term 六大都市 = 6 daitoshi and they are Tokyo, Osaka,
Kyoto, Nagoya, Kobe and Yokohama.

Is there are any exact rules when city has right to be called 大都市 and when it
is just 都市?  
For example, how many 大都市 in Japan? Only 6 or more? What about Sapporo, is it
daitoshi or just toshi?
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meaning word-choice words usage nuances
How to determine 大都市 vs 都市
大都市 is just a plain word "large city" that doesn't have a strict definition in
Japanese. And the problem here is that 大 in **大** 都市 and **六大** 都市 are
different in meaning.

By saying `Ｎ大[noun]` with a number _N_ , you mean "the _N_ major [noun]s",
where the 大 roughly means "important" in this formula. They are not
necessarily "large" in measure or whatever.

> 世界三大料理 _The Three Grand Cuisines of the world_  
>  テニス四大大会 _the four majors (Grand Slam) of tennis_  
>  五大湖 _The Great Lakes of North America_ (there are five)  
>  六大栄養素 _the six major nutrients_  
>  etc.

Japanese (and East Asian languages) is generally fond of enumerating "[the _N_
best things of the
kind](https://ja.wikipedia.org/wiki/Template:%E5%9C%B0%E5%9F%9F%E4%BB%A3%E8%A1%A8%E3%83%AA%E3%82%B9%E3%83%88)"
with this expression. The number that comes before is written in kanji more
often than not.

Related: [Is there a way to say the two
best?](https://japanese.stackexchange.com/q/68280/7810)
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Today, 大都市 typically refers to large cities with at least 1M population, but
the criterion is subjective and relative. There is no strict definition
defined by a concrete number. When the term [六大都市 was
determined](https://ja.wikipedia.org/wiki/%E5%85%AD%E5%A4%A7%E9%83%BD%E5%B8%82)
by the government in 1922, Nagoya and Yokohama had only 0.4M people. 札幌 had
only 0.1M people in 1922 but has 1.9M now. I believe most people don't
hesitate to call 札幌 a 大都市 now, but this is subjective.
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