You say that as if that's all America is. America controls a vast amount of land. It's the third largest country in the world and, unlike the first two largest (Russia and Canada), most of that land is also usable for either farming or resource extraction. But that's not the only reason it's an empire. It's also an empire because it has a large military presence in multiple countries who rely on it to supply defense at least in part. But another reason it's an empire is that it has vassal states. It doesn't matter how small those states are. Especially not when they are in strategic Pacific locations.