Infants deploy two cognitive systems that support numerical thinking: one for representing small sets of objects (<4) in parallel, and another for representing large, approximate numerical magnitudes (>4). Despite infants’ demonstrated numerical competence, preschoolers fail to show which of two sets contains more elements until they have acquired some number word meanings (e.g., Brannon & Van deWalle, 2001). However, previous studies mixed up small and large numbers. The current study asked whether preschoolers’ failure depended on the number of elements in sets. Two- to 4-year-olds were shown either small or large numbers of rectangles, and were asked to choose the set that had more. We analyzed children’s performance based on their number word knowledge. Results found that children were better at comparing small than large sets, and numeral acquisition facilitated numerical comparison. Implications for the role of the two cognitive systems and number word learning in numerical comparison were discussed.