Also, the surplus in nearly all the periods of ancient Rome, was still largely an agrarian surplus, extracted either from slaves, or from feudal workers / colonates in the territories outside the city.
The city / empire survived not by its own products and a commodity-producing economy, but by feeding an agrarian surplus off its many colonies.




Imperialism can occur in any class society. In its most general definition, it means the theft of land, labor, and resources of a weaker country to feed a stronger one. So we do call it "Roman imperialism":
The surplus here is an agricultural one.
Imperialism takes a different shape under capitalism, where instead of a landed aristocracy / slave-owning class doing the colonizing, its finance capitalists in the imperial core exporting production to low-wage / underdeveloped countries to produce commodities cheaply.