It's not just the "heavy NP": relative phrase length modulates the production of heavy-NP shift.

Heavy-NP shift is the tendency for speakers to place long direct object phrases at the end of a clause rather than next to the verb. Though some analyses have focused on length of the direct object phrase alone, results from two experiments demonstrate that the length of the direct object relative to that of other phrases, and not the length of the direct object alone, predicts production of the shifted structure. These data support an accessibility-based interpretation of length effects in word order emerging from incremental production processes, in which longer phrases tend to be less easily planned and therefore are delayed during utterance planning.