The need to service populations of high diversity in the face of high disparity affects all aspects of network operation: planning, routing, engineering, security, and accounting. We analyze diversity/disparity from the perspective of selecting a boundary between mice and elephants in IP traffic aggregated by route, e.g., destination AS. Our goal is to find a concise quantifier of size disparity for IP addresses, prefixes, policy atoms and ASes, similar to the oft-quoted 80/20 split (e.g., 80% of volume in 20% of sources). We define crossover as the fraction c of total volume contributed by a complementary fraction 1 - c of large objects. Studying sources and sinks at two Tier 1 backbones and one university, we find that splits of 90/10 and 95/5 are common for IP traffic. We compare the crossover diversity to common analytic models for size distributions such as Pareto/Zipf. We find that AS traffic volumes (by byte) are top-heavy and can only be approximated by Pareto with alpha = 0.5, and that empirical distributions are often close to Weibull with shape parameter 0.2-0.3. We also find that less than 20 ASes send or receive 50% of all traffic in both backbones' samples, a disparity that can simplify traffic engineering. Our results are useful for developers of traffic models, generators and simulators, for router testers and operators of high-speed networks.