Back in the times when we counted in bytes or kilobytes, programs were simpler to manage. One person could overview all code. Now we have programs with million lines. Nobody can oversee that.
https://github.com/chrislgarry/Apollo-11
Reviewing it would be non-trivial, but in terms of absolute code size, it's not much larger than a moderately-popular open-source project today. It's significantly smaller than React, for example:
https://github.com/facebook/react/tree/master/src
Edit: Downloaded both repositories. 'wc' puts the AGC at roughly 60K LOC, while just the 'src' directory in React is 70K.