Purpose: This study explored the relation between therapy group size and language outcomes for children receiving school-based language therapy through an implementation science lens. Method: Data for the current study were gathered as part of the Speech-Language Therapy Experiences in Public Schools study. Participants included 273 English-speaking kindergarten through second-grade children with language impairment receiving business-as-usual therapy on the caseloads of 75 speech-language pathologists. Data were collected over an academic year, including weekly therapy logs, speech-language pathology questionnaires, and pre-- and post--language measures. Results: Descriptive analysis revealed that children primarily experienced small-group therapy sessions (two to four children); however, there was considerable variability in group size. Hierarchical general linear modeling indicated that caseload size did not explain group size variability. However, the number of student cancellations was positively associated with receipt of large-group sessions. Notably, a significant negative association was found between receipt of large-group sessions (i.e., five to 10 children) and language outcomes. Children who received more than the average number of sessions in large groups (i.e., more than 5% of total sessions) experienced 0.18 SD less language gain over the academic year compared to the mean (0.54 SD). No other group size configurations (i.e., individual, small group, and extra large) yielded significant associations with language outcomes. Conclusions: The findings suggest that young children receiving language-based therapy in large groups make substantially fewer language gains over an academic year. These results have considerable implications for educational policy and clinical practice, which are discussed through an implementation science frame.