As project size increases, communication needs to be supported. The point of most methodologies is to reduce communications problems, and a methodology should live or die on its merits as a communication facilitator.
All other things being equal, productivity will be lower on a large project than on a small one.
All other things being equal, a large project will have more errors per thousand lines of code than a small one.
Activities that are taken for granted on small projects must be carefully planned on larger ones. Construction becomes less predominant as project size increases.
Scaling up a lightweight methodology tends to work better than scaling down a heavyweight methodology. The most effective approach of all is using a "rightweight" methodology.
