This is accurate. Syracuse probably overtook Athens as the largest of the Greek cities by the late 4th century BC, and kept growing right down to the Roman siege and sack of the city in 212-210 BC.

Syracuse always had a large agricultural hinterland; among the Greek states, the size of its territory was matched only by that of Athens and Sparta. This allowed it to produce a large surplus of grain. Syracuse grew rich off of trade in grain and wine with the other Greek cities of Greater Greece and the mainland, as well as with the Carthaginians. This gave it the means to defeat repeated Carthaginian expansionist efforts as well as the famous Athenian invasion of 415-413 BC. Its population is estimated to have already been equal to that of Athens by the time of the Sicilian Expedition. Some time after the defeat of the Athenians, the Syracusans extended their city wall to include the heights above the city that had been at the centre of the fighting; with this extension, Syracuse became one of the largest cities by walled land area in the ancient world. However, much of the space within the walls was not inhabited. The reason for the spectacular growth of Syracusan wealth and population in the late 4th and early 3rd centuries is a bit more complex.

Unlike the city-states of mainland Greece, the states of Sicily were ruled by tyrants for most of the Classical period. These tyrants had the peculiar habit of trying to enlarge their own power base by forcibly moving subjected populations to their own urban centre. This allowed for easier recruitment, taxation and physical control. The history of Greek Sicily in this period is therefore one of constant mass migrations, with Greeks and indigenous peoples moving to and fro as the fortunes of war determined. Greek tyrants and Carthaginian settlers constantly vied for power over the parts of the island they didn't already control. By the mid-fourth century, the long reign of Dionysius I of Syracuse - most successful of the Sicilian tyrants - had left much of the island desolate, with Syracuse as perhaps the only remaining major urban centre. Wave after wave of displaced populations and enfranchised mercenaries had been settled in and around the town.

This did not mean, however, that it was a densely populated city. Incessant wars had impoverished the island, and many of its previous inhabitants had been killed, sold into slavery, or forced to flee. Agricultural production and trade had collapsed. The successors of Dionysius I presided over the smouldering ruins of Syracuse's wealth. It is said that the grass grew tall enough in the marketplace to pasture horses there, and that citizens hunted for wild boar in deserted suburbs.

At that point (around 340 BC), Corinth, the mother city of Syracuse, sent over a small army under the command of one Timoleon to restore democracy to the city. Timoleon was startlingly successful. Not only did he manage to unite the remaining population of Sicily in support of his democratic project, ousting tyrants and establishing democracies as he went, but he also won a crucial victory against yet another Carthaginian invasion army at the river Crimissus in 339 BC.

With peace restored to Sicily and constructive efforts under way to form a system of government that the varied groups that made up the Syracusan population could live with, it now seemed that Syracuse was once again a good place to settle. Our sources tell us that as many as 60,000 Greeks migrated to Sicily. Grain production rose again, and trade networks were reestablished. Syracuse prospered once again.

Despite later troubles, such as the invasion of Pyrrhus of Epirus and the defeat of the democracy and return to tyranny under Hiero II (in 275 BC), it seems the city continued to grow. For most of the 3rd century BC, it was a staunch ally of Rome, which helped keep it neutral in the major conflicts of the time. Thanks to an unprecedented half-century of peace, Syracuse became the richest and most populous city in the Greek world.