Particle aggregation plays an important role in many marine biogeochemical processes such as determining the vertical flux of particulate material and trace metal scavenging. Models of particle aggregation vary in complexity and in this paper I compare the behavior of a detailed size spectrum model with that of a series of simple, two size class models with different representations of aggregation, all of which have appeared in the literature. The simplest model uses a first-order representation of aggregation kinetics, while two other models have nonlinear representations of aggregation. The simplest model is unable to reproduce the dynamic or steady state behavior of the size spectrum model. Results from the two nonlinear size class models show better agreement with the behavior of the size spectrum model. I find that the mode of aggregation and the dependence of aggregation on particle size are crucial for understanding the differences between the models.