Students need to create mental models for different types of addition and subtraction situations in order to develop a broad and viable understanding of these operations. Although most students succeed when changing or combining sets, situations that demand a quantitative comparison of sets seem considerably more difficult in the first school year. Textbooks represent the most important learning resource for elementary school mathematics teachers. However, research on their impact on students' knowledge is limited. Hence, we examined textbooks' role in students' ability to model quantitative comparisons by analyzing the learning opportunities presented by four German textbooks for Grade 1 and by conducting a secondary analysis of a dataset based on 1513 students from 84 classes that used one of these textbooks. The results revealed differences in the textbooks' topic-specific instructional quality as well as a significant relation between this quality and student achievement in quantitative comparisons.