Low-consistency (LC) refining is used in mechanical pulping and in general to give a final touch to a papermaking pulp. The desired—and undesired—effects come from changes in the property and size distributions of the different kinds of fibrous particles. In our study, we focus on the changes in the size distributions of mechanical pulps, measured with an optical fibre analyser. Pulp samples were collected before and after industrial LC refiners of thermomechanical pulps with spruce as raw material. We demonstrate that changes in the observed size distributions of fibre length and diameter can be reproduced with a stochastic model with just two parameters for particles breaking uniformly at random locations. One probability controls the breaks per unit length that shorten fibres, and another the splits per unit diameter that generate more fines. Our findings support the hypothesis that these two processes are separate so that breaks in length do not govern the increase in fines. Both fibre shortening and fines generation increased with refining energy but, at a given energy, only fibre shortening showed clear differences between trials. In a two-stage refining trial, the probability that fibres shorten was equal to the product of the single-stage probabilities. In addition, the two-stage probability fell on the same straight line as the one-stage probabilities when both were plotted against the refining energy measured from the threshold energy at which breaks start.