For some reason, I tend to get hired to work on large code bases, even though I've met many people who seem to have those perfect memories and instant analytic abilities which make them a natural for said code. Ultimately, as your code grows, complexity management skills become extremely important. This is why I have a number of modules I've written or contributed to heavily in order to make this easier:
Seriously, folks. As you move through your programming career, start paying attention to complexity management. When something is overly complex, figure out how to automate or simplify it. You'll usually gain flexibility and comprehensibility. Complexity management is your real job.