To understand why this isn't happening for the Unigram model, recall that under the Unigram model, all tokens in a hypothesis are fully independent whereas the collocation and the Bigram model assume dependencies between tokens.
under the Unigram model a solution involving a large number of items is automatically discouraged because it can only work with marginal probabilities, whereas the other models without syllable structure can "abuse" their respective notions of context to capture dependencies not only between words but also between subword units.