We study self-interstitial cluster migration properties, such as dimensionality of the motion and activation energy barrier, as functions of the cluster size, by means of molecular-dynamics simulations in bcc-Fe. The atomic interactions are described using a recently proposed potential, fitted to reproduce self-interstitial atom (SIA) configuration energies in close agreement with the results of ab initio calculations. We show that this potential provides a dynamic migration energy for the single SIA in agreement with the experimental value. We also show that, in the case of clusters formed by up to five SIAs, the migration energy decreases with increasing cluster size, but remains higher than previously believed. This is the consequence of the change of the migration mechanism of these small clusters from purely three dimensional (3D) to preferentially one dimensional (1D) and of the fact that these clusters take different configurations during migration, including anomalous ones. While the concept of fast 1D diffusion of large SIA clusters remains valid, the obtained results suggest a revision of both the rapidity and the dimensionality of the motion of small interstitial clusters.