It makes sense to me that lawyers in large departments tend to specialize more. They see more examples of any given issue than do Jill-of-all-trades generalists in small departments. The more experience you accumulate, the quicker and more skillfully you can handle a new variant, which is another way of saying that you are accelerating on the experience curve toward greater productivity.
Expertise should build up with experience if the lawyer consciously attends to learning (See my post of Sept. 1, 2008: learning methods with 12 references.). The resulting improvement is known as the learning curve (See my post of Nov. 6, 2006: delegate to go up the efficiency curve; May 31, 2006: the sigmoid curve of management initiatives; Feb. 2, 2008: specialist billers at law firms and learning curves; Oct. 10, 2006: core competencies and skill curves; July 15, 2006: Horndal effect; June 20, 2007 # 1: Horndal effect of increasing productivity; May 21, 2008: lesson of the experience curve from manufacturing; Nov. 6, 2006: expert lawyers work hard to get there; and May 23, 2008: use less outside counsel as experience deepens.)