Average boulder mass 〈m〉_B and the physical spread for a mass-dependent collision rate Λ(m, m′) = Λ₀ (m + m′) for different boulder-grain particle mass ratios: (a) analytical model as given by Eqs. (B.19), (B.21); (b) RPMC simulation. The simulation uses n = 1 792 RPs divided into n_B = 256 boulder-mass RPs and n_g = 6n_B = 1 536 grain-mass RPs with equal-weight swarms, M_i = M_j∀i, j ∈ {1, …, n}. The boulder masses in (b) are averaged over 10 runs; the filled area indicates the physical spread of the mass distribution, and the error bars indicate the standard deviation of the mean boulder mass over 10 runs. The horizontal black line indicates the expected final boulder mass . The total mass ratio of grains and boulders is , so boulders are expected to grow to 7 times their initial mass, as indicated by the horizontal black line. The horizontal axes refer to dimensionless time .