So you claim that beyond some magic size limit a codebase cannot be easy to understand. Well, quantify it. At what number of lines of code does a codebase pass from the domain where it is possible to easily understand it to where it is impossible?

I suspect you didn't pay any attention to what I wrote about how people go about ensuring that large codebases maintain readability. That saddens me a bit because it means you probably aren't interested in learning anything.