A network device includes circuitry and multiple ports. The circuitry is configured to hold a definition of a normalization function that determines, based on (i) a reference probability of applying a congestion indication operation to packets having a predefined reference packet-size and (ii) a packet-size parameter, a normalized probability of applying the congestion indication operation to packets whose size equals the packet-size parameter. The normalization function depends exponentially on a ratio between the packet-size parameter and the reference packet-size. The circuitry id configured to store packets in a queue, and to schedule transmission of at least some of the queued packets via an output port, to calculate the normalized probability for a given packet, by applying the normalization function to an actual reference probability and an actual size of the given packet, and randomly apply a congestion indication operation to the given packet, in accordance with the normalized probability.