  We study size and growth distributions of products and business firms in the
context of a given industry. Firm size growth is analyzed in terms of two basic
mechanisms, i.e. the increase of the number of new elementary business units
and their size growth. We find a power-law relationship between size and the
variance of growth rates for both firms and products, with an exponent between
-0.17 and -0.15, with a remarkable stability upon aggregation. We then
introduce a simple and general model of proportional growth for both the number
of firm independent constituent units and their size, which conveys a good
representation of the empirical evidences. This general and plausible
generative process can account for the observed scaling in a wide variety of
economic and industrial systems. Our findings contribute to shed light on the
mechanisms that sustain economic growth in terms of the relationships between
the size of economic entities and the number and size distribution of their
elementary components.
