Even at a larger size, though, it can still work – but the need for effective communication and diplomacy is even greater. You also want to be sensitive that you probably do not have all the context you might need when reviewing another project's code, so humility is important. When I do this, it's usually in the form of clarifying questions, suggestions, or requests.
I think it's actually because when you're in a young fast-growing company, the success of the company is literally everyone's responsibility. You have both the means (because the company hasn't yet ossified into management structures and the codebase is small enough that most people can be familiar with all of it) and the incentive (because a large portion of your compensation is in stock options that are only worth something if you succeed) to materially affect the company's prospects. And many people who join in that environment don't get the memo about when it becomes inappropriate for a new, larger structure.