Some embodiments of the invention provide a data-plane forwarding circuit (data plane) that has a flow-size detection circuit that generates flow-size density distribution for all or some of the data message flows that it processes for forwarding in a network. The flow-size (FS) detection circuit in some embodiments generates statistical values regarding the processed data message flows, and based on these statistical values, it generates a FS density distribution that expresses a number of flows in different flow-size sub-ranges in a range of flow sizes. In some embodiments, the density distribution is a probabilistic density distribution that is based on probabilistic statistical values that the flow-size detection circuit generates for the data message flows that are processed for forwarding within the network. The FS detection circuit in some embodiments generates probabilistic statistical values for the data message flows by generating hash values from header values of the data message flows and accumulating flow-size values at memory locations identified by the generated hash values. In some embodiments, the generated hashes for different data message flows can collide, which results in the accumulated flow-size values being probabilistic values that might have a certain level of inaccuracy.