Previous cross-cultural studies of social and dimensional comparison processes forming academic self-concepts (the big-fish-little-pond effect (BFLPE) and Internal-external frame-of-reference (I/E) models) have mostly been based on high-school students and two subject domains. Our study is the first to test the cross-cultural generalizability of both comparison processes across reading, mathematics, and science by combining of the TIMSS and PIRLS 2011 databases (15 OECD countries, 67,386 fourth-graders). Consistent with the I/E model, high achievement in mathematics/reading had positive effects on self-concept in the matching domain but negative effects in the non-matching domain. Extending the I/E model, students engaged in assimilating comparisons between science and reading (i.e., achievement in one subject had positive effects on self-concept in the other) but contrasting comparisons between mathematics and science. Strong BFLPEs (negative effects of class-average achievement on self-concept) were found for mathematics but were smaller for reading and science. The results generalized well across all countries.