We propose a user model that explains the shape of the distribution of file sizes in local file systems and in the World Wide Web. We examine evidence from 562 file systems, 38 web clients and 6 web servers, and find that this model is an accurate description of these systems. We compare this model to an alternative that has been proposed, the Pareto model. Our results cast doubt on the widespread view that the distribution of file sizes is long-tailed; we discuss the implications of this conclusion for proposed explanations of self-similarity in the Internet. Keywords: File sizes, lognormal distribution, long-tailed distribution, self-similarity. 1.