If you can factor out that big long function into ten smaller functions that are each useful in isolation, then your code is probably going to be a lot easier to understand, regardless of how your structure your commit history. Future code reuse is also easier. Testing each function in isolation may be simpler as well.
Once you do all of that, putting each of the standalone functions in its own commit is trivial, and each commit can have a more focused commit message. Reviews are easier too, since you can easily view each function and its associated docs/tests/etc separately.
Local optimization on these sorts of metrics are pretty much always going to lead to perverse outcomes.
Again, this is a straw man. Nobody is saying to blindly optimize to the metrics, or that this is a black-and-white thing where every commit needs to fit within a hard line limit. But, all else being equal, I've personally found that my code quality improves when I try to break things into smaller independent changes. I also find it much easier to review PRs where other people do that.
