  Established Open Source Software (OSS) projects can grow in size if new
developers join, but also the number of OSS projects can grow if developers
choose to found new projects. We discuss to what extent an established model
for firm growth can be applied to the dynamics of OSS projects. Our analysis is
based on a large-scale data set from SourceForge (SF) consisting of monthly
data for 10 years, for up to 360'000 OSS projects and up to 340'000 developers.
Over this time period, we find an exponential growth both in the number of
projects and developers, with a remarkable increase of single-developer
projects after 2009. We analyze the monthly entry and exit rates for both
projects and developers, the growth rate of established projects and the
monthly project size distribution. To derive a prediction for the latter, we
use modeling assumptions of how newly entering developers choose to either
found a new project or to join existing ones. Our model applies only to
collaborative projects that are deemed to grow in size by attracting new
developers. We verify, by a thorough statistical analysis, that the Yule-Simon
distribution is a valid candidate for the size distribution of collaborative
projects except for certain time periods where the modeling assumptions no
longer hold. We detect and empirically test the reason for this limitation,
i.e., the fact that an increasing number of established developers found
additional new projects after 2009.
