>How can you not be able to find your way through a startup codebase if you came from a corporate job, where presumably you worked on much bigger projects?

You'd be surprised. Enterprise might mean tons of legacy visual basic apps, or crappy add-ons to some proprierary platform, not "much bigger projects". Just because you e.g. work for a branch of Unilever somewhere, it doesn't mean you work on their payroll system or anything close to it.