Structuring 1M line of code as one function or 1M functions is clearly equally absurd. What is the sweet spot? There must be an answer from psychology and/or information theory.
The answer is: What is the problem? It depends on the problem.
We're unlikely to encounter a problem that is solved with a single funtion, or as one function per line. But the important thing is to look at what the code should achieve. Most importantly, local decisions should be made based on what the code should achieve in a global context. Local syntactic optimizations are less important than the global picture.
I strongly agree with the Golang authors and with experienced C programmers that it's much better to write a few lines more and be more clear and explicit in exchange. Note that there are some additional lines that increase complexity, but I would argue that lines which contribute to clarity do not contribute complexity. In fact, those investments in additional lines usually decrease the number of moving parts.
Syntactic homogeneity is important so one can easily see what one piece of code achieves in the global context. It does not help if we micro-manage and constantly think about the type of for loop or lambda abstraction or error handling mechanism to use, only to shave another line off.
Unfortunately looking at the actual problem is what most people forget. Instead the discussion are about languages (filter, maaap. maaaaap), frameworks, libraries, object orientation without any concern of what these features can do towards reaching a specific goal.
Now that you mention information theory, I want to mention the term "Semantic compression", created by Casey Muratori. I think he has done at least one stream about it, which you should be able to find on YouTube. In general I recommend to follow him. He is one of the most experienced and no-nonsense guys I've found on the internet.
Keep things 'square'. This is a fuzzy concept and I don't really know how to explain it properly, other than that the effort spent on each layer of abstraction should be roughly equivalent.
Your example is a single 1m-line function or 1m one-line functions. In this case you probably want 100 functions of 100 lines each (and yes, refactoring like this you probably save 100-fold in overall LoC so it works out.) And your 100 functions are probably nested in a 10-deep hierarchy where they all do roughly the same amount of cognitive work.
