The charge sharing effect has been investigated in a 300 μm thick silicon pixel detector bump-bonded to a Medipix2 readout chip and exposed to heavy ionizing particles of various energies, namely protons and alpha-particles. The lateral spread of the charge carriers from the interaction of a heavy ionizing particle in the silicon detector causes a sharing of the charge among adjacent pixels, giving a signal in clusters of multiple pixels of the Medipix2 device. A model of charge sharing including the plasma effect on top of diffusion and drift processes has been compared to the measured lateral spread of the charge for bias voltage values beyond the full depletion voltage. Results of the measurements of the cluster size as a function of the energy and incidence angle of incoming heavy ionizing particles are also reported and compared with the model.