Just look at it, it's teensy. I'm pretty sure you could fit the entire city inside the Academy grounds.
It's especially odd since the other cities in Remnant are quite large. Just look at Atlas!
Seriously, zoom in and you'll see that most of those buildings are skyscrapers.
And while there aren't a lot of good shots that show off Vale's size, we can still see that it easily eclipses Vacuo.
Look at the 2d shadow buildings that make up the skyline and you see how far the city extends.
Also, it had an entire goddamn freeway within the city walls.
Heck, I'm about 99% sure that even Argus, which isn't even its own kingdom, is bigger than Vacuo
It just makes Vacuo come across as kind of pathetic compared to the other kingdoms. It feels more like an overgrown village than a city.
And why would you pick this tiny place as the location for your last stand against Salem?!