Capitals are usually among the largest cities in their regions; often they are the biggest. For example, Montevideo is Uruguay's capital as well as its biggest city. The capital city may also be the most important center of commerce, as in London or Bangkok.
However, a capital is not always the largest city in a country. For example, the capital of the India is New Delhi, which is smaller than Mumbai; and the capital of Pakistan is Islamabad, which is smaller than Karachi. Also, in countries with subdivisions like the United States, the capital cities or towns of the federated states are often not the largest or most populated town. For example, New York City is the biggest city in the United States and in New York but is not the capital of either.
An unusual case exists in Canada; the federal capital, Ottawa, is not the largest city in its province, Ontario, which is Toronto. But Toronto happens to be the capital of the province of Ontario. This way, Toronto is both a capital and a non-capital, depending on how you see it, as a city in a country or part of it.