Finding a balance between Tall and Wide is probably a tough ask for most 4X. I do think that Humankind did not fail to find balance, it feels it did not even try to look for it. The game always pushes for more: more territories, more cities, more population, more money, more science, more, more, more etc. and rewards it very well. It makes playing with few big cities far from optimal and one easy experience shows very well that two middle-sized cities are better than one double their size: it suffice to merge them and immediatly see the two flourishing cities become one unique with unemployed people and not enough food to feed everybody. Part of the reason for the lack of food is the animal barn. Having two cities, you get two of those and their disconnected from proper scaling yields. Merging lets only one exists and results in famine many times.

I want to address those animal barns but I also want to put in the same bag the forge and the coal plant. They give a flat bonus per, respectively, horse, copper and coal. I find their yields too massive for very, very little investment in the case of the animal barn and forge, reaching, on big maps, +30 or +40 in many cases for a pitiful amount of production giving a lot to any city, even the smallest ones.

To fight this, without discussing the extraordinary importance of horses and forges (copper stays a very important resource nowadays but forges are less a thing of many contemporary nations), I'd suggest the following:

1. the number of times the bonus given for horses, copper and coal would is awarded is hard capped by the number of territories composing the city.

This way, a bigger city would benefit from more horses, copper and coal, reflecting also its needs, while smaller towns would have to do with less. Before being accused of forgetting the role played by population, I'd mention that part of the growth revolved around getting those extra horses and that populated cities tend to grow in territories too. Plus, it is more a question of gameplay than anything else here.

There are three similar infrastructures: Nuclear plant, Financial district and Inventor's Workshop that give a % bonus based on uranium, oil and saltpetre. It may seem weird to put them into the mix, their bonus being not a flat one. I'd see no objection however to have their bonus limited in the same way, for consistency reasons.

I hope all that made a bit of sense and wish you a very pleasant weekend

Perfect moment to use this.
But indeed, finding a balance is quite a thing between small and large cities.

To be fair to Humankind, the fundamental game framework of attaching territories to a city to make it larger (and leaving territories unattached outposts) is in direct opposition to the "More More More!" style found in most other 4Xs.  Look at the ugly and tedious "city tile spam" that goes on in Civilization and most other 4Xs.  Endless Legend's city spam is far prettier and more satisfying than Civ's, but you still need to use cities to claim important territories and resources.  Humankind on a fundamental level changes this:  Where you might fill a space with a dozen cities in another 4X, you may have only 2-4 large cities in Humankind.

Yes, merging cities doesn't work very well because population demands grow exponentially and stability problems grow linearly, and merging large cities eliminates "per-city" .  This is a perfectly handpicked example of "fewer cities bad", but if you grew that large city from the ground up, your metropolis wouldn't be in that mess.  (I almost never merge cities for this reason)

Would I like some more mechanisms for Tall Empires?  Sure!  You and I have each made a few posts about it.  But if for you to say "Humankind doesn't even try to look for balance in tall empires" seems pretty unfair to the dev team; look at globe-spanning empire in Humankind made up of eight enormous cities, and then look back at a globe-spanning empire from Civilization 5/6 whose 30 cities have small territies pressed against one another with mathematically-tiled precision and see where Amplitude has subtly moved the needle.

@Spacesuitspiff: Agreed with most of your post but I do feel that your not taking into account the part that makes wide the obvious efficient choice: fame. Having more cities, instead of bigger ones is better for gaining population (and so in a way for waging wars), producing influence (flat bonuses+population gains), science (as some of the bonuses are related to population: libraries, museum, etc.), gold (same kinda things), creating districts (cost goes up too fast with big cities), even attaching territories can become tough (buton that topic, creating new cities can be quite costly). So, all the stars are easier to get for wide players.

@RedSirus:  Well, I do disagree, although I think I do understand why our opinions diverge. I don't think the mechanic of territories makes it less of a „always more“ game. I do not really wish to push it into a comparison between HK and CIV. I have thousands of hours of CIV but haven't played much of the VI and comparing would not be fair. I do feel the map is completly taken in HK faster and that territories give the impression that there's not one bit of terrain left, even in the harshest deserts (icy or hot). In comparison, I found ES2 to be less grab everything, as small systems with puny one or two planets would be lefts alone if they do not produce strategic resources. I found grabing land in Civ to be less an „ugly tile spam“ than what I do in HK. I tend to get as much land as possible. If some territories end up not being attached, it is more a question of optimisation than of logic. Playing tall is possible in Civ (at least in the V) and you don't need to get expansion stars, which give a very strong incentive to grab land just for grabbing land. It is also weird to me to have a world with only 50-80 cities at best on a 10 players map and they tend to be an „ugly district spam“ kinda Coruscant-like way too often.

So, merging is a mess. I feel, reading your paragraph, that you simply urge me not to merge cities and to do a bit of planning to avoid the issue. I'd rather keep the possibility and have the system corrected. That said, merging was only an example to showcase the issue with some buildings. I wish to offer an idea to correct part of the issue.

I may be a bit unfair to the devs, and do apologize if they feel I've been overboard. However, if I do feel they spent a lot of time creating a beautiful game, implementing many mechanics that gave a fresh air into static systems and pushing most of what they could to make the game better and better, I also believe that in doing so they obviously noted that they made Wide win versus Tall and, if they missed it, I'm sure their playtesters mentionned it. I understand why it could have been left in that state but I do also believe that not much as been done since release (a year ago now) to compensate for the imbalance in favour of Wide.
Spending time in countries with old towns, settled centuries ago, you tend to feel that cities pressed against each other is in those part of the world a natural state of urbanisation (look at most of Europe). Cities in Civ were not necessary US-square-placed in my (many games). I could say that I do feel HK pushes towards succession of territories that do not feel natural in many cases (cities cut by moutains, city plazza far on one side of the city, etc.)

At least, the good thing is, you enjoy it and I'm glad for you.