Etruscan decline

[Jim Webster]

But just a thought, you probably don't need immigration for the city scaling, but rather just suck up under-utilised labour from rural areas.
I think

For much of history you needed immigration from rural areas into the city to help maintain population levels. The greater mortality due to unhealthy surroundings seem to have meant that cities couldn't maintain population without this

[aligern]

Perhaps  Etruscan colonisation policy played a part. They seem to have generated colonies a lit earlier than Rone and that might siphon off population  so that the earlier cities did not just grow.  Another point might be choice of site. Were early aetruscan towns on the sort of tight steep hill site that say Volterra is?  That would result in smaller cities than one spread around seven hills.
Another point may be simply the distance that Etruscan cities were along the maturity curve compared to Rome.  Roman aristocrats were , to start with, bandit chiefs with clans of herdsmen . If the Etruscan nobility had long ago moved to very controlled blood lines and marriage within the class they might well have been more exclusive tgan Rome's relative meritocracy.  A lot if tge reputation of Rome's  noble houses is probably a backward projection from later times?
Roy

[DBS]

Perhaps  Etruscan colonisation policy played a part. They seem to have generated colonies a lit earlier than Rone and that might siphon off population  so that the earlier cities did not just grow. 

Possibly, though we do not know whether there really was any sort of Etruscan colonisation in truth, as opposed to cultural influence and perhaps the movement of individuals.  To be fair, Cato thought that the Etruscans colonised, and may well have had a sound basis for that belief, but equally the main beneficiaries of colonisation, whether Roman model or Greek model, tended to be the less wealthy, who left the metropolis in hope of getting a decent plot of land - "levelling up" opportunities, as it were, albeit at the expense of another city or hill tribe.  Which should not have weakened significantly the effective and useable military force of the metropolis.

I am sure that you are right about the initial choices of site playing a factor - as Nicholas has observed, size begets size.

There is also the question of whether the Etruscan cities were more vulnerable to fluctuating trade routes.  In the early period, it was the southern cities which seem to have been most powerful and had the largest size; they were well placed for trade with the western Mediterranean and Magna Graeca.  However, they decline in later periods - perhaps squeezed by the Romans and Carthaginians, as well as being more directly faced with Roman military competition - and the northern cities seem conversely to have prospered, perhaps controlling trade routes up into Europe or across to the Adriatic.  Though their late prosperity would not be enough for them to put on sufficient muscle mass to pose a meaningful speed bump to Rome...

[Dangun]

How unusual was the location of their cities? Anecdotally they seem to have been more pre-occupied with defence than more southerly cities. But I imagine there is knowable data on this?