Although I can't answer the original query directly, there are a few observations to make, following up on the reasons for support for or against the amendment as noted in a prior response. Of the 12 amendments originally proposed for the Bill of Rights, this is the only one that is pending. The last 10 proposed were ratified as the first 10 amendments, while the second proposed - after 200 years - was finally ratified, as the most recent amendment.
Madison
Madison's argument, seen in the context of all the times following, is actually the most forceful. He expressed a concern about the districts getting too large. Today, this is 750,000 people per representative, which equates to only 80 seconds per person per term. It would be a stretch of the terms "Democracy" or "Democratic Representation" to call any such arrangement either of these, or anything else but an oligarchy.
The amendment, which put a cap of 50,000 people per representative would essentially bring representation down to the level of individual neighborhoods in today's cities, and almost even down to subdivisions! It would give what, today, we refer to as small towns, separate representation.
Without the cap, with the ratio having ballooned up beyond 750,000 to 1, you now have very limited time per person (80 seconds). This is a bottleneck. As such, individuals must be funnelled to pass through it, this funnelling taking the form of large collectives - usually connected to individual issues or causes that have found large enough collective following to register enough above the clamor of other issues to be heard and recognized.
These are called Special Interest Groups.
Also, without the cap, districts are no longer intimately tied to small cohesive population groups, as much as they would be if individual neighborhoods or small towns could be separately representated. This leads to a winner-take-all situation, where many different groups vie for that one representative. To remedy this requires designing districts, and even twisting and contorting districts, to try and embody population blocs within them.
It can also be applied in the opposite direction to lock out population blocs, either intentionally or just by the sheer power of numbers. For, you also have to consider the situation where a smaller population bloc is distributed throughout many districts, either because the districts were designed that way or simply because they're desegregated and live in many different places, rather than being concentrated in one. It gets no representation.
Both cases have, as their lead symptom, Gerrymandering.
Larger districts also tend to push everything toward a smaller number of larger parties. Smaller parties, especially those that are regional, would be drowned out and would be unable to win any district election. This contributes to the devolution toward a two party system.
James Jackson
It appears that his primary contention was simply that if the ratio were made too small and the numbers too large, the legislature would become too unwieldy. In particular, at the extreme case of a 1 to 1 ratio, it was a logistical impossibility to have everybody in the country meet. For this reason legislatures pool constituencies into representatives.
I don't know how much foresight there was about the future growth of the country, though there seems to have been some, with there having been discussion about putting a cap on the future size of the legislative body.
Logistics isn't as much of a concern today, as it was then. Given the immediacy of contact that most people have, today, one could begin to entertain the possibility of not only adding a 50,000 cap, but removing the 30,000 lower bound and even permitting it to go all the way down to 1 to 1; i.e. open invitation for participation by adult citizens on any and all legislative proceedings.
But there is one issue, that must have arisen in the context of discussions opposing a cap on people to representative ratio, that I don't see any mention of, and despite the potential ability to overcome the problem of logistics for having small ratios, it would be highly relevant even today!
The Balance Of Federalism
The United States has a bicameral legislature to serve the needs of federalism. It is a federal union, rather than a national republic. As a way to balancing between the states and people, one body was provided to represent the states, the Senate; and the other to represent the people, the House. This is reflected by the fixed number of Senators per state and by their method of selection. They are currently chosen by the state on a state-wide ballot, and in earlier times by the representatives of the people - the state's legislature. A vestige of the earlier directly-by-the-state method still exists in that replacement senators may be appointed by the state, rather than by ballot.
The compromise between the people and the state is also reflected in the electoral college and how the President and Vice-President are chosen. One part, which may be linked to the Senate, has two electors per state, while the other, which may be linked to the House, has one elector per district; with D.C. latter added in by treating it as if it were a state with representatives - so it currently has 2+1.
The key issue is "fixed". If a 50,000 to 1 cap were enforced, then the House would balloon up to nearly 7000, an average of 140 per state, while the Senate remains fixed at 2 per state.
How are you going to assign 140 electors per state? That almost certainly would not be happening on a winner-take-all basis! Instead, it would probably have evolved into the arrangement Nebraska and Maine already have, that more naturally reflects the allocation of electors: 1 for each district, 2 on a state-wide ballot, just like the House and Senate.
And more importantly: what about the ratio?
At a 140 : 2 ratio, the balancing provided by the electoral college between the people and the states breaks down, and the states are drowned out by sheer numbers. The electoral college reduces effectively to one vote per district.
I think the founders saw that and at some point said "We have to put a cap on the House's size" - just for that reason alone. This tends to skew the debate toward Jackson's side.
So, I suspect this is one of the reason why there was discussion, at the time, about putting a cap on the size of the House. Without a cap, you would have to also allow for incremental expansion of the Senate, to maintain some kind of balance between the two. Otherwise, you lose the balance of federalism that is ingrained into the method for choosing the President and Vice President.
As a compromise, then, they agreed to leave the issue unsettled, to keep the first proposed amendment out but to keep any caps on the size of the House out of the constitution, as well ... deferring all further deliberation on these matters to future times, like with the cap.
Perhaps with a little more research, we can find traces of such discussions, as well as a better tracking of who the respective advocates were. The link in the earlier reply, to the debates on the issue ("https://memory.loc.gov/cgi-bin/ampage?collId=llac&fileName=001/llac001.db&recNum=375"), is broken. In its place I could only found Founding Fathers’ Debates Today, which discusses the issue of balancing (the Madison versus Sherman debate) for representation and federalism, but I can't find anything discussing ratios or House size caps. Likewise, I found Electoral College Explained, but nothing that discusses anything like the 140 : 2 imbalance that would arise from having a 50,000 : 1 ratio cap.
If I find other references or links, particularly those connecting to the question of the first proposed amendment and the alignment the founders had with respect to it, as well as to their debates, I'll add them in a later edit.
