Even though the Kingdom of Italy and the Kaiserreich came into existence at roughly the same time, the two states were quite different, hence different outcomes to unification. In short, although Germany was (and still is) a highly diverse polity, the Kingdom of Italy faced a far more daunting task at uniting the peninsula into a coherent economic whole.
The economic gap between Northern and Southern Italy is pretty apparent in most scholarship on Italian economic history. Although some historians have qualified the divide as more complex than a north/south disparity, few economic historians would characterize Southern Italy as more advanced and prosperous than the North. There were some exceptions to this economic underdevelopment. Kingdom of the Two Sicilies built the first railway in the peninsula and sponsored some proto-industrial projects, but these as a whole were the exception rather than the rule. Most of the economic indices in 1861 demonstrate a preexisting divide between the North and South. The Southern poverty rate ran at 52 percent compared to 37 percent in the Center and North. Even though Italian life expectancy was quite low by contemporaneous European standards (about thirty years), the South's life expectancy in 1871 was two years and two months shorter than the their other counterparts. Similarly, adult literacy rates were much lower (13 percent versus 39 percent) than the rest of the country. Southern industrial development prior to unification also lagged behind the rest of the country. There were very few railways and little investment in infrastructure like hydroelectric power. Such industries that were in the South lacked room for growth.
The gap between North and South yawned ever wider in the decades after unification. This was partly the fault of the Northern-dominated central state. Liberal Italy tended to avoid the developmental projects that would have alleviated this gap. However, one of the main causes of the South's economic malaise was the global drop in grain prices and the rise of trade protectionism in European states like Germany. This meant that the cornerstone of the South's economy, agriculture, became increasingly uncompetitive both within Italy and on a European level. Thus the main export of the South became its own population, feeding the global Italian diaspora.
The diaspora from the South feeds into the perception that Italy was predominantly an agricultural state. Industry in the northern peninsula achieved impressive growth rates in the decades after unification, a development that parallels the Kaiserreich's economic growth after unification, the Gründerzeit. State protectionism encouraged Italian industry helped to foster the growth of firms like Fiat (automobiles) and Ansaldo (shipbuilding) which were quite modern. One of the problems for Italy though was that uneven internal economic development lessened the internal market available for Italian industry. This ensured that industrial growth was hostage to global market forces. Italian firms were quite adept at adapting to foreign technology and manufacturing processes, but had to adapt them to fit into Italy's domestic markets. The relatively poor and underdeveloped agricultural sector meant there was little impetus from within the country during the latter-half of the nineteenth century to adapt modern technology. Italy's relative lack of natural resources exerted a further restraint on industrial growth. Italian politics also played a role in limiting the overall growth of Italian industry. Italian industrial entrepreneurs and the state existed in a highly entwined relationship that guaranteed both state contracts and bailouts due to the importance of firms like Breda to the state. While this relationship between industry and politics was not unprecedented, it did stifle Italian industrial efficiency and acted in conjunction with the other factors outlined above. All of this prevented Italian industry from branching out throughout the peninsula.
The Kingdom of Italy's military mirrored the story of Italian industrialization. In certain areas of military development caused considerable consternation to foreign contemporaries, such as the battleship Caio Duilio or Regina Marina's early adaptation of the torpedo. Despite these advanced examples, the Kingdom could not modernize its forces en toto due to budgetary restraints. The North/South split added a further dimension to this problem as the South was a source of manpower, but Southern recruits were less educated and less healthy than their Northern counterparts.
What all of this amounts to is that the Kingdom of Italy was a great power of Europe, but it as arguably the least of the great powers. The dilemma for Italian statesmen was to how to use Italy's relatively weak power in the most decisive manner possible ("weight in the scales"). Italian policy did not have the realistic option of acting independently, thus Italian foreign policy of the Liberal Era often appears to be quite opportunistic.
Sources
A’hearn, Brian. "Institutions, externalities, and economic growth in southern Italy: evidence from the cotton textile industry, 1861‐1914." The Economic History Review 51, no. 4 (1998): 734-762.
Cohen, Jon S., and Giovanni Federico. The Growth of the Italian Economy: 1820 - 1960. Cambridge [u.a.]: Cambridge Univ. Press, 2001.
Toniolo, Gianni. The Oxford Handbook of the Italian Economy Since Unification. New York, NY: Oxford University Press, 2013.
Zamagni, Vera. The Economic History of Italy, 1860-1990. Oxford: Clarendon Press, 1993.