The reality is that there are no more long lists of bishops for any city in the Empire besides those just mentioned in Irenaeus, Eusebius and Epiphanius. No one ever recites a list for instance of the bishops of Antioch or Alexandria so my guess is that Hegesippus only produced two lists - i.e. one for Jerusalem and one for Rome. The bishops of Rome because as he says he happened to be there and Jerusalem out of a discussion of the death of James who is said to be their first bishop and the subsequent heresies that emerge from the choice of Symeon as his successor:
The same author also describes the beginnings of the heresies which arose in his time, in the following words: “And after James the Just had suffered martyrdom, as the Lord had also on the same account, Symeon, the son of the Lord’s uncle, Clopas, was appointed the next bishop. All proposed him as second bishop because he was a cousin of the Lord. Therefore, they called the Church a virgin, for it was not yet corrupted by vain discourses. But Thebuthis, because he was not made bishop, began to corrupt it. He also was sprung from the seven sects among the people, like Simon, from whom came the Simonians, and Cleobius, from whom came the Cleobians, and Dositheus, from whom came the Dositheans, and Gorthæus, from whom came the Goratheni, and Masbotheus, from whom came the Masbothæans. From them sprang the Menandrianists, and Μαρκιανισταί, and Carpocratians, and Valentinians, and Basilidians, and Saturnilians. Each introduced privately and separately his own peculiar opinion. From them came false Christs, false prophets, false apostles, who divided the unity of the Church by corrupt doctrines uttered against God and against his Christ.
The same writer also records the ancient heresies which arose among the Jews, in the following words: “There were, moreover, various opinions in the circumcision, among the children of Israel. The following were those that were opposed to the tribe of Judah and the Christ: Essenes, Galileans, Hemerobaptists, Masbothæans, Samaritans, Sadducees, Pharisees.” [Eusebius Church History 4.22.4]
Now what is so interesting about this list is that Irenaeus seems to take an interest in the author's list of bishops in Jerusalem and Rome but his list of heresies does not include many of these names.
The υπομνηματα later ascribed to 'Hegesippus' (a corruption of Josephus) CAN'T be thought to have included a detailed catalogue of each one of the aforementioned 'heretics.' It just doesn't make sense that Irenaeus would hold up this work in AH iii.3.3 but then develop a completely different list of heresies. As I said there can't be much more to the description of these heresies other than the list of names that appears above.
So now we are at a loss to explain how the υπομνηματα could have filled up FIVE BOOKS of information as Eusebius notes if it was just a 'memoir' of a second century Christian. The opening words of Eusebius's discussion say that the υπομνηματα "has left a most complete record of his own views." This is a very generic statement which ultimately tells us nothing about the book.
Eusebius goes to mention that in these υπομνηματα "he states that on a journey to Rome he met a great many bishops, and that he received the same doctrine from all." This is reminiscent of Irenaeus's allusion to the same work being written by Polycarp and the reference to the controversy in Corinth which follows is also common to both traditions.
I guess my point is that I don't see enough in any of these descriptions of the work to see how it could have filled up five volumes. The υπομνηματα CAN'T be the kind of work described by Schaff - viz. "the work appears to have been nothing more than a collection of reminiscences covering the apostolic and post-apostolic ages, and drawn partly from written, partly from oral sources, and in part from his own observation, and quite without chronological order and historical completeness."
I think that most of the material just cited appeared as an introduction to the main work. Just look for instance at the words "being in Rome, I composed a catalogue of bishops down to Anicetus." This implies that the author is referencing the five books as a separate work so too the idea that though the book was written during the reign of Anicetus the preface comes from a period AFTER that date - viz. 'and Anicetus was succeeded by Soter, and he by Eleutherus. In every succession, and in every city that is held which is preached by the law and the prophets and the Lord." As I noted in my previous post, I think this sounds remarkably similar to the phraseology of Irenaeus in AH iii.3.4. Whoever wrote these words they were written as an introduction to the main work from which they are clearly separate.
Does that mean that the five books could have been a heavily Christianized ancestor to our Jewish War and Pseudo-Hegesippus? Yes, I think so.
