The problem in the tendency to ascribe “imperialist” status only to the few most powerful nation states is that it applies a national frame of reference to a global, and crucially, class based phenomenon.
To quote Marx in the manifesto, chapter one:
The modern state is merely the executive committee for managing the common affairs of the whole bourgeoisie.
The whole bourgeoisie – because the bourgeoisie is an international class. Borders are for us, not them. They go where they like, they get educated wherever they like, they invest wherever they like, they employ people wherever they want, they own land and property wherever they damn well choose. There is no self contained capital, the containment is just for the people who generate it with their labour. The fact of bourgeois internationalism is more true now than it has ever been – yes there are wars between nations and individual fractions of the bourgeois class may root for one side or another, but as any capitalist will tell you – competition is good for capitalism… from a global, class based perspective the international bourgeoisie win every war and the working class lose them all. Capitalism is a totality, and the imperialism which defines this epoch of capitalism is also a totality. Imbuing imperialism with a specific nationality and looking at it from the perspective of individual “national” capitals instead of global capital is a mystification which obscures it’s nature as a global social formation borne out of the capitalist mode of production, a mode of production which has been international in scope from its earliest stages of development.