Festering Code Bases and Budding Code Bases:

Some teams produce what I call a festering code base. In a festering code base, the team changes the code primarily by adding code to existing methods and adding methods to existing classes. The results are predictable. Classes and methods grow malignantly, eventually becoming thousands of lines long.
Better teams produce budding code bases. Developers create new classes and methods and delegate work outward. Periodically, they collapse structure back into a simpler form, but the dominant trend is to grow the code by creating new structure.

I'd never put much thought into how code bases grow in the past. Feathers has some interesting ideas here about the characteristics of good and not-so-good growth and how languages and tools might promote good growth.