My company's product is roughly the same number of lines of code as Windows XP, and it's written in M (AKA MUMPS). You manage.
Really, working on a big project is more about good quality control and good documentation than anything else. You can do big projects object-oriented or procedural, and as long as you keep standards high there's not a huge amount of difference.
You ever try a pink golf ball, Wally? Why, the wind shear on a pink ball alone can take the head clean off a 90 pound midget at 300 yards.