The Big Empty

This footnote is pretty heavy on the world-building, and absolutely non-essential if you just take what you read without question. For those who want the details and explanations, however, here we go.

Let's start with some math. If Canterlot is forty miles away from Ponyville (chapter 8), and it takes them a little more than an hour to get there (chapter 17), that means their airship is going near 35 miles per hour. If that's the case, and it takes them around six days to go from Canterlot to Manehattan (chapter 19), that means there's between 5,000 and 5,100 miles of wilderness they're passing over (around 8,000 kilometers). For reference, that's the distance from the northernmost point of Mexico to close to the center of Alaska. That's pretty far.

So why is Equestria so full of empty space? The answer lies both in Equestria's history, and the way pony society is structured. When Celestia and Luna were ruling, after the deposition of Discord, they had to help the country develop and recover. Being an entity who is primarily concerned with chaos, Discord didn't do much to help Equestria develop its technology, its culture, its science, and so forth. It was basically anarchy until Celestia and Luna came along. When they did, it was only around a thousand years later that Luna became Nightmare Moon, at which point they were both ruling from their castle in what is now the Everfree Forest.

When the Nightmare Moon incident happened, the resulting uproar led to ponies being much more individualistic and separatist, in terms of social and economic policy. It was a way to ensure each town and city would survive, should Celestia also turn (they had no way, at that point, to know whether she would prove to be just as unstable as Luna). Because industry and culture were so closely tied to their towns, it wasn't until significantly later that intercity commerce became important. The reason Manehattan is so much bigger is because it had access to all the water of Starlight Lake, as well as a fairly direct route to the oceans in the north. Generally, though, there was no need for huge, sprawling cities, nor roads between them.

From a social and cultural standpoint, it's more about the gender ratio. With only one male to every ten females, procreation is not nearly at the runaway pace it is for us humans, so space is not as big a concern as things such as education, resources, and so on. Moreover, with most ponies being raised in towns that are already fairly isolated, the need or desire to explore and develop the rest of the world never comes about. And, again, because ponies aren't born as often as we are, there's a much larger sense of community. Friends and neighbors play just as important a role in foals' lives as their parents.

To clarify, none of this has much to do with town size, only the space between cities. Canterlot is pretty huge, and Manehattan is bigger; size and population density have more to do with access to resources than anything else.