The more rural you get, the more likely it is that the fire trucks have been purchased secondhand from urban fire departments that can afford to buy new equipment prior to the existing equipment literally falling apart (I've got an extended family member that runs the fire service for an entire county in a western US state, and often see him when he stops at my house while driving new-to-them equipment from the east coast).

You've also got a lot of volunteer departments where each firefighter keeps their equipment in their own personal vehicle.

Why firetrucks in the US are so large is a good question. In the spirit of Chesterton's Fence, I'd assume that there is a good reason until proven otherwise.

That's utterly impossible. It's the same reason America can't have public transit: since it doesn't make economic sense in small remote towns of 200 people, then it can't be done in huge cities. I can't explain the logic, but this really is how most Americans think.