At medium ice concentrations, sea ice consists of separate floes of different sizes interacting with each other through inelastic collisions, in a way similar to two-dimensional polydisperse granular gases. The dynamics of this type of ice cover is poorly understood. In this paper, a molecular-dynamics sea-ice model based on simplified momentum equations and a hard-disk collision model is used to analyze processes of cluster formation in sea-ice floes. The clusters, formed due to size-dependent equilibrium velocities of floes under a given forcing, have statistical properties dependent on the average ice concentration and on the parameters of the floe-size distribution. In particular, in terms of the size of the largest cluster in the system, two regimes are observed: one at low and one at high ice concentration. At high ice concentration, the dominating cluster spans the entire model domain and contains the majority of floes. The exponent of the cluster-size distribution increases with increasing exponent of the floe-size distribution. The results are discussed from the point of view of the collisional contribution to the internal stress in the ice, as well as from the role of clustering in the floe-formation processes. Thus, they may contribute to the formulation of more reliable sea-ice rheology models valid at medium ice concentrations.