The Aeneid is an epic poem, and it's full of exaggerations-- that's the point of that kind of poetry. Nonetheless, the ancients were convinced that lead melted in flight-- Aristotle, for instance, mentions it.
The Diodoros is different: it's a narrative historical source, that purports to be factual. It's written perhaps 250 years after the event, but uses an earlier source (perhaps Timaios). But the text, as all ancient historians, was transmitted by hand copies over centuries, so something could be wrong with the text (for instance, it could have read "half a mina" rather than "a mina", i.e. stones 250g in weight); Diodoros could have misunderstood his source; his source could have gotten it wrong. But here's the passage-- which makes it sound like Baleares *usually slung very heavy
http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Diodorus_Siculus/19F*.html109 But when Hamilcar saw that his men were being overpowered and that the Greeks in constantly increasing numbers were making their way into the camp, he brought up his slingers, who came from the Baliaric Islands and numbered at least a thousand. 2 By hurling a shower of great stones, they wounded many and even killed not a few of those who were attacking, and they shattered the defensive armour of most of them. For these men, who are accustomed to sling stones weighing a mina,19 contribute a great deal toward victory in battle, since from childhood they practise constantly with the sling. 3 In this way they drove the Greeks from the camp and defeated them.