In this research we analyze the main drivers of cities' growth according to their sizes (Mega, Big, Midsize and Small) applying a cross-country analysis that involves about 114 countries and 1920 cities in a period of time of forty years (1960-2000). We have tried to verify if the growth of cities is linked to urbanization processes that encompass economic structural transformations (performing urbanism), or if their growth is linked to dynamics that disadvantage further economic development (non-performing urbanism). Once considered the relation between the evolution of cities sizes and the shape of the urban structure, we analyze the quality of this structure as a ?new? deep determinant of economic growth. This is why, in the last part of the research, we look for possible relations between the quality of the urban structure and economic growth.Our results indicate that growth in very large agglomerations tends to be ?non-performing? and that the urban structure which fits better with economic growth is characterized by high urbanization rates but low hierarchical structures.