  We introduce a simple agent-based model which allows us to analyze three
stylized facts: a fat-tailed size distribution of companies, a `tent-shaped'
growth rate distribution, the scaling relation of the growth rate variance with
firm size, and the causality between them. This is achieved under the simple
hypothesis that firms compete for a scarce quantity (either aggregate demand or
workforce) which is allocated probabilistically. The model allows us to relate
size and growth rate distributions. We compare the results of our model to
simulations with other scaling relationships, and to similar models and relate
it to existing theory. Effects arising from binning data are discussed.
