Five experiments explore categorization and category-based congruity effects in mental comparisons. The first four experiments concentrate on categorization of infinite-set small items. The experiments vary the additional items presented, and whether those items appear once (Experiments 1-2) or repeatedly (Experiments 3-4). Additional items include other small items (Experiment 1), relatively large items (Experiments 2-4), and items involving non-size dimensions (Experiment 4). The critical small items show a complete congruity effect only in Experiments 1 and 3. These results suggest that categorization of infinite-set items may be based on range information alone (Experiment 1), but that multiple categorizations based on multiple ranges (Experiment 2) may require attentional effort. The results implicate categorization as a central process in mental comparison, despite differences in ease of categorization across paradigm.