The voice system of Tagalog has been proposed to be symmetrical in the sense that there are no morphologically unmarked voice forms. This stands in contrast to asymmetrical voice systems which exhibit unmarked and marked voices (e.g., active and passive in German). This paper investigates the psycholinguistic processing consequences of the symmetrical and asymmetrical nature of the Tagalog and German voice systems by analyzing changes in cognitive load during sentence production. Tagalog and German native speakers' pupil diameters were recorded while they produced sentences with different voice markings. Growth curve analyses of the shape of task-evoked pupillary responses revealed that processing load changes were similar for different voices in the symmetrical voice system of Tagalog. By contrast, actives and passives in the asymmetrical voice system of German exhibited different patterns of processing load changes during sentence production. This is interpreted as supporting the notion of symmetry in the Tagalog voice system. Mental effort during sentence planning changes in different ways in the two languages because the grammatical architecture of their voice systems is different. Additionally, an anti-Patient bias in sentence production was found in Tagalog: cognitive load increased at the same time and at the same rate but was maintained for a longer time when the patient argument was the subject, as compared to agent subjects. This indicates that while both voices in Tagalog afford similar planning operations, linking patients to the subject function is more effortful. This anti-Patient bias in production adds converging evidence to “subject preferences” reported in the sentence comprehension literature.