Group threat theory understands prejudice as a manifestation of the threat, either actual or assumed, that minority groups pose to majority groups. This theory is often operationalized by analyzing the impact of group size on anti‐immigrant prejudice. We test this hypothesis with a new dataset documenting 487 effects of group size on prejudice provided in 55 studies. More than half of these results show no relationship and the remainder shows both positive and negative relationships. Three explanations for this divergence are that there are (1) differences in the measurement of prejudice and immigrant group size across studies; (2) differences in the model through which size is hypothesized to lead to prejudice; and (3) differences in the geographic unit of analysis at which these relationships have been considered. Our analyses support the measurement explanation: results vary across studies because they reflect different measures of group size and prejudice.