Interesting that your reading on the Risorgimento would talk about the North-South divide in those terms. While the North was marginally wealthier than the South at the moment of unification, the really palpable differences during the lead-up to unification (the "Risorgimento") were actually institutional, rather than economic.

But that doesn’t mean we can ignore economic differences altogether. It's just difficult to quantify those differences because record-keeping was dramatically different across the Italian states. Fair enough that within these differences, there is also the undeniable fact that we have a lot more data points from the North compared to the South, which is in and of itself is a signifier of the South's less effective institutions. The consensus does acknowledge that these less effective institutions translated to less florid economic life, but we just can't reliably quantify how significant that economic impact was, especially because most of what economists mean when they say "Economic Growth" is really industrial growth, which pre-unification Italy had very little of across the peninsula: Italy was a late industrializer and an ineffective industrializer.

But net of peninsula-wide weak industrial growth, in addition to considerations on institutions, there is also the mere fact that the North was more urbanized than the South, which even in a pre-industrial economy means there was more value-adding activity going on (even if most of this value-adding activity was artisanal). The North was also proportionally more agriculturally productive (which is a necessary condition for urbanization, but not a sufficient condition) and in an agricultural economy, economic productivity is necessarily linked to agricultural productivity. While North's denser urbanization, higher agricultural output, and overall more florid economic life compared to the South doesn't really have a clear start date, we can say probably occurred gradually as the South's ties to the Greek Eastern Mediterranean noticeably deteriorated between the tenth and thirteenth centuries, causing the peninsula's center of population to move northward (in antiquity, the South had instead been Italy's most urbanized region, however by the Late Middle Ages the North was much more urbanized, and indeed was Europe's most economically vibrant region until the 16th century).

Some might speak of geographic advantages that the North benefitted from, but geography only really magnifies existing institutional issues: in antiquity, as long as the Empire could guarantee trans-mediterranean grain shipments the South had no barriers to urban growth (and besides, the South had some enormously fertile regions of its own: notably Eastern Sicily, inner Campania, and Puglia). But compared to the South, the institutional history of the North (for which we can go all the way back to the competing city-states of the Middle Ages in order to dissect) encouraged Northern landowners to be much more thorough in extracting as much value as possible from their land. This cycle of productivity would prove self-reinforcing, just as an example, canals for intensive irrigation can also be used to transport riverboats and encourage further economic exchanges, while those same pressures encouraging productive use of land would eventually nudge capital owners to invest in increasingly productive industrial ventures (another specific example as to how preexisting conditions 'nudge' each other is is that in the lead-up to industrialization the North's canals could also be put to use to power machinery).

I wouldn't want you to come away with the idea that there was no industry at all in Italy, just that compared to the most economically developed parts of Europe (England, parts of Scotland, the Low Countries, as well as select parts of France and Germany) industrial activity in Italy was much less florid, even in the more productive North. That being said, the part of Italy that showed the most industrial growth was undoubtably Lombardy, with Milan at its heart. Glimmers of early industrial growth also appeared in the Northeast, however it never really took off outside of Triest (in Austrian-held Italy, it appears Milan siphoned off much of the Venetian cities' excess financial and human capital) and the Veneto's economy would converge with the rest of the North very slowly. The Northwest, in spite of a somewhat haphazard catch-up policy, performed second best compared to Lombardy (and this is aggregating both Piedmont and Liguria). The rest of the peninsula offered glimmers of industry here and there, but nothing statistically substantial: in Tuscany the steelworks at Piombino were a small industrial node, while in the Kingdom of Two Sicilies the seaside city of Salerno saw some swiss investors try to kick off a textile industry, but neither was really able to catalyze any sort of widespread industrialization.

When did the Italians themselves recognize this gap? In the pre-unification period institutional differences were more important: we are still in a phase where the Piedmontese were taking steps to improve their sputtering industrialization, but the North hadn't perceptibly taken off. Most other Italians outside of Lombardy were not really thinking of economic policy. Even in Piedmont, where economic concerns were largely driven by the need to keep pace with the Austrian Empire (of which prosperous Lombardy was a part, little over forty miles from their capital of Turin) an "institutional" narrative was much more commonplace. An significant example of the "institutional" nature of Piedmont's political dialogue is that the Kingdom of Piedmont-Sardinia was willing to provide a safe haven for subversive activists from other parts of the peninsula. These activists would pressure the Piedmontese government to intervene in the affairs of other Italian states with a wider objective of achieving some sort of unity (ill-defined until fairly late in the Risorgimento, but still a significant phenomenon: just as an example, in the summer of 1848 the serving mayor of Milan, Gabrio Casati, would not only defect to Piedmont but also be appointed Prime Minister shortly after his arrival) and also by that same process these activists kept the Piedmontese political narrative outward-looking, sparing Piedmont itself from much of the unrest present in the rest of Italy (the most significant example would be activists in Piedmont organizing the Expedition of the Thousand to topple the Bourbon Monarchy. Consider the fact that there was no activity of any kind the other way ‘round).

Once unification was achieved, the country began a more earnest march towards industrialization (which still wouldn’t match the most productive parts of Europe until after the Second World War). While the biggest single spurt of industrialization disproportionally favoring the North would only occur during the First World War, once unification was achieved the triangle formed by the cities of Milan, Turin, Genoa nonetheless established itself as the country’s industrial heartland very quickly. As early as the 1880s an active parliamentary debate emerged on a “Southern Question,” whereby the country’s political and social narrative explicitly acknowledges the South’s lesser economic development, and kicked off over a century of policies directed to closing the gap.

I've actually written on the topic a few times in the past. You might be interested in a few more in-depth answers of mine from the past, of which this is the most recent. I cite fairly specific estimates of economic development from the instant that unity was completed (when unified estimates first become available) in this other answer which might also interest you. I also wrote this somewhat related answer on Italian economic stagnation which might clear up a few other questions.