This is because frequent words, such as function words, are part of many MTUs and therefore have a very high out-degree, e.g., the neuron representing "the" has over 82K outgoing edges.
we obtain classes for words or MTUs using a version of Brown-Clustering with an additional regularization term to optimize the runtime of the language model (Brown et al., 1992;Zweig and Makarychev, 2013).