Research on peer reviewing has revealed that comments received from peer reviewers are helpful when it comes to making revisions in an individual's writing, but the role of providing comments to peer writers has been little explored despite the potential value of such research. In this study, we explored how student reviewers learn by reviewing peer drafts in the context of reciprocal peer reviewing. Undergraduate students in an introductory physics course participated in this study as part of their course activities. Participants wrote technical research drafts, reviewed three or four peer drafts, and revised their own drafts in the SWoRD system. A total of 3,889 comment segments were analyzed in two dimensions: (a) evaluation (strength vs. weakness) and (b) scope (surface, micro-meaning, and macro-meaning). We found that providing weakness comments for micro-meaning and strength comments for macro-meaning improved the reviewers' writing qualities. In addition, reviewers' initial writing skills and the quality of reviewed peer drafts influenced the types of comments given. The results are discussed, along with their implications for improved writing through reviewing.