So with this information in hand, it seems very unlikely that Rome reached even half the population that is commonly thrown out there.

No it doesn't. McEvedy's understanding of the sources is incomplete at best. There is no argument anymore. I mean there are plenty of debates about ancient demographics that are still ongoing, but none of this is a part of that. These arguments were presented by an amateur (but decently well respected) demographer back in the 60s, and the debates have moved on and left McEvedy behind.

In sum, McEvedy is wrong. It's pretty easy to demonstrate this by only a cursory examination of his argument, and if I were inclined (I don't have his text on hand) to be more thorough it probably wouldn't make any difference because the argument is so clearly incorrect.

First, McEvedy doesn't understand how the grain distributions worked, and doesn't seem to know the texts. The argument about who exactly was getting the public grain is still a little bit of an open debate, but it's a debate about whether the grain was distributed to adult males or only to the adult male heads of household. It's a bit of a complicated debate, and I don't actually need it to deal with McEvedy's argument. McEvedy's numbers for the number of recipients of the free public grain are the numbers that were fixed by the end of Augustus' lifetime, which presents some serious problems. First of all, McEvedy seems unaware that the Augustan numbers were reduced significantly. When the Lex Clodia extended the subsidized grain to free grain in 58 a famine occurred in 57, for reasons that are not entirely clear to us, and so Pompey was given control of the cura annonae in 57 in order to stabilize the supply. Precise figures are hard to come by, but the usual estimate is at least 300,000 recipients of public grain by the end of 57, possibly as many as 320,000, which is the figure that Suetonius gives. The public grain was changed over time. Caesar reduced it from 320,000 (according to Suetonius) to only 150,000, tasking the praetors with keeping an up-to-date list of the most destitute citizens and adding them to the roll when a spot was opened by death. So much for McEvedy's argument that the city would have starved if grain was not being distributed to every single individual (which doesn't work, we know that the private market accounted for the bulk of the city's food supply, which was supplemented by, not predicated on, the public grain): even by his own logic there was a time when that was obviously not happening. QED. To further put the nail in the coffin we can continue, however. Augustus in the Res Gestae gives us nice clear figures. Augustus lists off several of his early examples of largitio, and mentions that all of these gifts were distributed to more than 250,000 people. Augustus tells us that in his twelfth consulship (5 BC) he distributed to 320,000 of the urban plebs some HS 240 per person. That's only the urban plebs, so on top of that we've got to add on the senatorial and equestrian population and the slave population. Which, by the way, McEvedy does not take any into account the divisions in the city's population. If the maximum population of Rome was only 200,000, based on the Augustan grain distributions, then there were no slaves in the city, there were no wives in the city, there were no children in the city, and there were no foreigners in the city. Why do I know that? Because we know that the public grain was distributed only to citizens, a point that was politically important. Cicero tells us that in 58, as Clodius' law was entering effect, slave owners in the city began manumitting their slaves en masse. Since freed slaves at Rome became citizens, and freedmen were expected to enter into an agreement with their former masters to remit part of their income or produce--the idea, says Cicero, is to have all these newly freed slaves collect their public grain and then give a portion of it back to their former masters, thereby allowing their masters to collect more than the allotted 5 modii per month. So first of all slaves are certainly being ignored here in the population figures. But if public grain was only being distributed to citizens--which is certain, one way or another, the texts are quite clear on this--and it was being distributed to all citizens, regardless of age or sex, and that the figure of the grain distribution equals the population of the city, then there are no slaves or foreigners in the city. That's...not really a claim worth addressing...

There's more that could be said about the grain argument, and I could be a lot more thorough about it, but I'm going to move on. McEvedy's on slightly better ground with the insulae argument, but it doesn't work. The information he's using is the Regionary Catalogues, or notitiae, of the fourth century, by which time Rome had actually experienced a significant population decline, so they're only used by demographers to fix a minimum population. The notitiae list buildings in each of the Augustan regions of the city using the formulaic verb "continet," followed by a list of buildings and neighborhood magistrates, beginning with any temples and public buildings and ending with the total measurement of the region. It's possible, and it's been argued before, that the insulae listed on the notitiae are not full buildings, but the argument is strained. First of all, the notitiae say "insulae." They do not say "partes insularum" or "cubicula insularum" or anything like that. They literally say "apartment blocks." If that's supposed to mean units within apartment blocks the usage would be utterly unique in the entire corpus of Latin literature, and wouldn't make a lot of sense--an insula is literally an island, because the building, being unconnected to other residences, resembles an island in space. The argument might be made that the notitiae list insulae off in the nominative, whereas the fomula "continet..." requires the accusative, which is elsewhere used. That's torpedoed by the fact that if insulae is supposed to be a genitive it needs to be plural insularum (unless there's only a single, gigantic, cyberpunk-style mega-insula in the entire region) and by the fact that the notitiae frequently mistake the nominative for accusative (e.g. vici for vicos). Further, it's illogical. When the notitiae list buildings they're clearly listing off the entire building, not parts of it. This is most apparent with the temples and public buildings, which are listed individually, but it's also apparent from the storage houses (horrea) and bakeries (pistrina) listed, which cannot be broken down into units. Domus are also present in the notitiae. The notitiae, immediately under every listing of insulae, list the number of domus in the region. There is certainly no chance, linguistically, that domus might be used to refer to a unit within a house rather than the entire house. Impossible. And even if we imagine the impossible, we are faced with the problem of trying to divide into standardized units private urban mansions, which is dodgy at best. Unless for some reason insulae alone are getting some special, unprecedented, treatment, it seems safe to assume that these are entire buildings that are being counted. As a final note, McEvedy's certainty about the space that these insulae would take up is misplaced. The Forma Urbis, which heavily fragmentary, seems to support in broad strokes the general information of the notitiae, and in any case we're quite certain that the city at its height was extremely (and dangerously) densely developed.

McEvedy is wrong. There are lots of debates in Roman demographics, but McEvedy's argument is based on a combination of imperfect understanding of the sources and a lot of arguments that are illogical even sometimes within the evidence that he is aware of. Indeed, most heterodox demographers of the city these days argue for a higher population in the city than has been typically suggested, not a lower one, and certainly nobody today argues for a population so ridiculously low as is being offered here.