Instructional videos have been increasingly critical to afford effective learning experiences in online courses, but most videos follow a lecturing monolog delivery format. This format tends to lead students to observe the videos passively. Previous laboratory studies have indicated that observing dialog videos of an instructor tutoring a tutee enhances student learning more than watching monolog videos of the same instructor lecturing. This paper describes two empirical studies that replicated the laboratory findings in large-scale college-level online STEM courses. The results show that observing dialog videos of tutoring led to superior learning outcomes compared to monolog videos of lecturing, regardless of whether students observe the videos individually or collaboratively, as long as they engaged generatively with the materials. This finding was confirmed across two tested STEM domains. The findings of these classroom studies suggest that observing dialog tutoring videos is a novel and robust online instructional format that can be generalized across STEM domains. The benefit of overhearing tutorial dialogs requires students to engage in "Constructive" and "Interactive" behaviors, as defined by the ICAP framework, rather than exposing students to long didactic lectures.