The ability to compare plays a key role in how humans learn, but words that describe relations between objects, like comparisons, are difficult to learn. We examined how children learn size comparison words, and how their interpretations of these change across development. One-hundred-and-forty children in England (36-107 months
68 girls
majority White) were asked to build block structures that were bigger, longer, smaller, shorter, or taller than an experimenter's. Children were most successful with words that refer to size increases. Younger children were less accurate with smaller and shorter, often building bigger structures. The dimensional aspect of taller emerged gradually. These findings suggest that children's interpretation of the meaning of size comparison words changes and becomes more precise across development.