Someone needs to state the obvious for those who miss it. Agile software development is not a silver bullet for productivity. Agile is not something you "mix in" and yield the same result faster. Applying agile practices to the humongous project won't result in massive software at lightning speed. That's not the aim of agile.
Agile's main effect is a decrease in project latency, not an increase in throughput. Agile development means delivering minimal but user-confirmed business value sooner, rather than delivering maximal but doubtful business value once at the end. The hope is that by going to the trouble of more frequent releases, the software will grow gradually but surely, as actual usage not guessing motivates what the software needs to be. The economy of agile doesn't result in a greater quantity of software features per development time period, but it's certainly intended to result in a lesser number of wasteful software features per development time period.
This shift in perspective affects everybody's expectations, so agile development is more than a set of new habits for developers. Project managers need to break their world-changing ambitions into much smaller chunks. Users need to become more involved. Analysts need to laser-focus their requirement-gathering. Architects and modelers (may) need to recalibrate the balance between resource costs and the pursuit of perfection.
If a plane flight is like a software project, then agile development won't decrease the time it takes to go from Seattle to Boston non-stop. But it will change the non-stop flight into a series of short connecting flights, and by the time you reach Chicago you might realize that you didn't really need to go to Boston in the first place.