They dont have to keep all of Italy just neutralize Rome. Once that is done the only potentially serious threat to Carthage itself is removed.
Easy, grant the Etruscan league independence from the Roman Republic, perhaps a puppet state of some sort in Magna Graecia, maybe, and only directly annex the rest of Sicily. If the Romans try anything with the Etruscans or the Greeks, burn the city to the ground.
As for Britain I just used it as hypothetical example, as It too was trade/naval nation, Which formed a huge empire off the back of these thing. and all without a huge land army.
Of course technological and social development changes some things, but the material needs of empire dont change. I.E military-economic power.
Britain is why the time difference does matter. Britain's empire was a result of specific social and technological conditions that didn't exist in Carthage's time. The early British colonial ventures were, actually,
private ventures. Occasionally the king would have a hand in financing it and making everything legal (like the land-grant to William Penn), but the driving source of
this sort of colonialism is the proto-capitalism that was evolving in western and central Europe at the time.
Basically in this time period, major overseas trading powers (the Dutch, the French, the British, the Portuguese, the Danish and Swedish for a short while) would form
economic colonies. This actually bears a lot of superficial similarities to the actions of Carthage; she, too, was far more into economic colonies than downright imperialism (again, the Barcids adventures in Iberia were the work of the Barcids as people/a family, not of Carthage). Carthage, like the Phoenicians she was descended from, would drop a small colony somewhere and allow the natives to bring the goods for trade.
This highlights the differences between the two time periods. The financial structures and labor resources simply aren't there to support the same sort of exploitation missions the imperial powers of the 17th and 18th centuries could and did.
That all, of course, changed again with the industrial revolution. After that imperialism took on a whole new flavor. It's most noticeable in Britain. It's interesting to see, but the rise of the British East India Company in the Indian sub-continent closely parallels the rise of manufacturing in Britain itself. Now, I'm not suggesting some sort of cause and effect thing, but rather that the specific conditions of the rise of industry and full-on financial capitalism, combined with the sudden availability of a huge amount of resources of every sort, ready and waiting (but not willing) to be exploited for the benefit of the Metropol.
That completely changed views on how trade should be conducted. Previously you couldn't come take resources for yourself, you had to trade for it. You
could come try and take things for yourself, but that only works when the technological gap is as big as like was between, say, that of the colony of Virginia in 1730 and the native Americans they faced. The Dutch also had limited success with this prior to the industrial revolution, conquering a huge area spread through the Indonesian archipelago. You can find some other examples scattered amongst the accomplishments of the first wave of economic imperialist countries (the Dutch, the Portuguese, the Danes, etc).
What really changed everything is suddenly the mild technological and organizational advantage that had allowed the British to conquer the Indian sub-continent was multiplied a million fold with the dual triggers of industrialization, leading to a much greater ability to innovate new technology and then produce it in vast quantities, and then nationalism, which suddenly made it somewhat plausible to have standing armies large enough to garrison huge tracts of land.
So, western Europe suddenly has the means to conquer their neighbors (in the loosest sense of neighbor...) with relative impunity and the example of Britain (and several other, lesser examples) in India. Britain at least
seems to be doing very well with India providing both immense amounts of basic commodities which Britain herself could then turn into finished goods to be sold back to the captive market in India.
People put two and two together viola, you've go the New Imperialism of the 1880's.
Now, in contrast to Carthage, the technological, social, structural, organizational, and just entire situation is completely different. They're not 'comparable' in that way. As long as Carthage makes most of her money through trade, she'll act like a trading empire. Look up 'mercantilism' and you'll get the general gist of what her behavior should be (although adjust for the far more ancient time period).