TL;DR: tall is not viable because expansion is not penalized much. If there is space to expand, you need only answer two questions:
is it worth the opportunity cost, and
is it safe?
Allow me to try and theorize a bit. I am no expert, feel free to criticize this bit! Please back your criticism with facts and numbers if possible.
Is expansion good?
Global yield of all resources is a
decisive factor in winning: more production, growth, science, culture, faith, etc, means better chances to win all 5 victory types. One should ask oneself: how do I get the best yield?
More pop primarily (more buildings as well, but let's focus on pop for now). The more pop across any number of cities, the more yield.
You need to consider the
increasing marginal cost of growing a pop in a city (the more pop in a city, the more expensive it is to grow to the next pop). ==> Therefore, it is good to found a new city because you'll eventually get more pop than you would have if you did not expand.
Besides, Housing and Amenities mechanics make it so that expansion is more or less the only way
not to waste food yield. These are strong incentives towards expanding.
Is expansion bad?
Expansion can be mitigated in a number of ways. In Civ V, science & culture penalties, NW scaling cost, and the global happiness system were important factors that made expansion
decreasingly interesting. Besides, there were a ton of buildings, wonders & policies that had a
multiplicative effect on a given city's yield (for example, +33% Science from Universities): this sort of "bonus" makes the next pop in the city with the building yield more than one in a newly founded city.
What do we get in Civ VI as expansion impediment? As far as I can tell:
- increasing builders, settlers and districts costs
- pop drop when finishing a settler (meaning it's less expensive to make settlers in low populated cities?)
- some very rare multiplier effects. So far, I can only name two wonders, Oxford University and the Ruhr Valley, that have multiplicative effect on the city's yield.
So,
no direct penalty, only
an increasing opportunity cost of the settler vs. buildings or military.
If building a settler will eventually yield more than building anything else, then do it. Otherwise, don't.
Therefore, I don't think "Tall vs. wide" is really a question is Civ VI. The game won't be won by a player with 4 cities when there is another player with 12 cities (in Civ V BNW, the player with 4 cities probably had his chances).
This is my hypothesis. Having not played the game yet, I would love for someone to try and contradict this in a convincing way
