We extend the theory of the "tunnel model" to the case of aggregation phenomena in magnetic fluids, which is valid for strong magnetic fields. Our model takes into account cluster-cluster interactions and is used to investigate the aggregation phenomena in a uniform applied magnetic field. Furthermore, Monte Carlo simulations have been carried out to clarify the aggregate structures of ferromagnetic particles, using our cluster-moving Monte Carlo algorithm. The main results obtained here may be summarized as follows : (a) the number densities of particles have an influence on the sizes of clusters, namely, the larger the number density becomes, the longer the clusters that are formed, (b) the size of a cluster increases exponentially with the strength of the magnetic interactions between particles, (c) the number density of large clusters also increases as particle-particle interactions become large, and (d) the results of the tunnel theory are in qualitative agreement with those of Monte Carlo simulations.