It's possible for one city to have a greater number of people than another, but for that city to feel more livable and less crowded. It's more about population density and infrastructure than it is about numbers. The population of New York City is roughly equal to that of London, but New York City's land area is roughly half that of London, so that means that the population density in New York City is double that of London. Looking at the numbers alone, that might prompt someone unfamiliar with either city to surmise that New York City must be much more crowded than London, but New York is nowhere near as old a city as London so it was laid out according to a much more modern plan which greatly affects things like transportation. There are many parts of London where drivers must pull over to the side of the road to allow an oncoming car to pass. You'd never see that in New York City because the grid pattern of the streets makes it so that all major roads are two-way streets or clearly defined one-way streets. I can tell you from personal experience that driving in either city is quite a headache, but it's much easier in New York City due to the fact that the street network is predictable, with roads intersecting one another at right angles most of the time.
Both cities have areas filled with large apartment blocks. In London there are council houses and in New York City they refer to council estates as "projects." Both consist of large clusters of buildings housing a great many people, but the majority of the housing in both cities is made up of smaller multi-family dwellings and single-family homes.
Another factor is the geography of the area. London is bisected by the River Thames, and the city stretches out north and south of the river on either side. But New York City is situated on several islands and peninsulae, so there wasn't anyplace to build once that limited land area had been developed. There are very few private homes on Manhattan Island, which is the centre of New York City, whereas there are quite a few single-family homes not too far away from the centre of London. Because most of the boundaries of New York City are defined by bodies of water, the line demarcating where New York City ends and an adjacent area begins is clear. But in London, the city doesn't end so much as it begins to taper off. Both cities are divided into boroughs, but London's boroughs border other counties whereas three of New York City's five boroughs don't share a land border with any other municipality, except in one case where two border one another.
Another factor is that London is divided into "Inner London" and "Outer London." "Inner London" is much smaller, but the population density is much higher. In fact, when we compare Inner London to New York City in the same manner that we compared New York City to London as a whole, we see a lot of similarities. Inner London has a land area that's roughly half that of New York City, but a population density that's about the same. This means that both cities have wide disparities in terms of population density from one portion of the city to the next, and that both cities have areas that are very built-up and crowded and areas that are relatively less built-up and less crowded.
The quality of life in any city is going to be based on how these factors come together as a whole. South Korea is roughly 1% the size of The United States, yet both countries have half a dozen cities with a population of one million or more (though some Metropolitan Areas in the USA have populations that exceed one million people.) China, the world's most populous country, is home to more than 100 cities with a population of one million people or more.
Seoul and Tokyo are infinitely more livable than Jakarta or Manila. Why? Well, for starters, Seoul and Tokyo suffered severe damage in the last century and both were rebuilt following that devastation. In many instances, city planners availed themselves of the opportunity to make changes that would allow the cities to grow with time. London suffered considerable damage, but nothing on the scale of Seoul or Tokyo, so the changes were not as drastic. And New York City hasn't seen any notable damage at all, so there hasn't been any reason to make major changes in whatsoever. Jakarta and Manila are being flooded with people from the outlying provinces looking for work, so the population continues to swell and the cities can't keep up with the ever-increasing demand for housing, they can't accommodate the ever-increasing number of commuters, it's just an absolute mess.
Sydney is a lovely city. Like almost everywhere in Australia, the statistics never tell the true tale. On the books, the city has an area that is just positively immense, but the city itself is concentrated to a few built-up areas and the bulk of the territory that's considered Sydney is actually made up of neighbourhoods filled with nice homes where everybody has a yard and a driveway along a tree-lined street.
By world standards, there are no "cities" in Australia, if we're talking about urban areas that are home to enormous amounts of people that are just endless stretches of concrete sprawled out in every direction.
One really needs to see these things to understand them. Travel to China and India, Korea and Japan, see Singapore and Bangkok, see Mexico City and Los Angeles and get a real feel for each place and you will see that these places are not just dots on a map. Each has a character all its own.