I will try to dig this topic.
As a part of development team in big software corporation I could only say that expectations over quality and size of games has been raised last 2 years up to the sky, its nearly impossible to predict release date. Agile software development is not enough. It is impossible to make games with people's hands anymore. It is to much to do. There are huge graphic/story/dialog engines that are filled with huge amount of data - example for a secondary character such engine needs almost every possible position, look, style, gesture, mimics, pose, in all possible cloths and colours used. Those engines are the biggest effort to comply properly.
If the need of building such engine or big data storage occurs it can easy take few months of hard work of whole makers team to run it without issues.
Also big games means big bugs/issues andthis means big testing aprach. Final testing itterations also could take few months if need to cover whole project in like ~75%.