  Open-source software is a complex system; its development depends on the
self-coordinated action of a large number of agents. This study follows the
size of the building blocks, called "packages", of the Ubuntu Linux operating
system over its entire history. The analysis reveals a multiplicative diffusion
process, constrained by size-dependent bounds, driving the dynamics of the
package-size distribution. A formalization of this into a quantitative model is
able to match the data without relying on any adjustable parameters, and
generates definite predictions. Finally, we formulate the hypothesis that a
similar non-stationary mechanism could be shaping the distribution of mammal
body sizes.
