The appeal of a big city is potential to connect with lots of people from different backgrounds, skills and culture. This enables richer commerce and social interactions.
This doesn't happen without huge amount of commute and lots of social gathering places like offices, schools, restaurants, malls, bars, sports, gaming, park etc.
A city fundamentally has to make it possible for a lot more number of N x k connections – where N is city population and k is number of people any one person can potentially meet – for much larger N and much larger k.
THis is the fundamental difference between a big city vs a cluster of small towns/villages which are fairly disconnected from each other. Basically, towns and villages scale down both N and k.
Okay, so the question is how do we make it possible for such a large number of N x k commutes to happen? To support large N and large k the commute has to be faster and over longer distance. It has to solve for flexibility of hours of the day and also be accessible for young/old, men/women, kids, disabled etc. There are cost affordability considerations too.
In some countries, like US, they have social culture where young and old don't live together, those with kids have different lifestyle than those without etc. Then in others like India, young and old do live together along with kids.
Solving for high N x k for mixed demographics in a large urban sprawl is fundamentally more challenging.