>I've been writing software almost non-stop for close to 25 years, and I still don't understand most of the code written by others.

I think you nailed it. In my experience, the increase in complexity isn't necessarily inherent to the amount of code so much as the amount of abstraction used as code size increases.

I've worked on some large codebases that only used simple, well-understood and/or well-documented abstractions which didn't feel as nearly complex as other codebases with abstractions that were more complicated than they were worth.