Learning is social.
I think most would agree that learning occurs through social interaction, but I see this as being different than learning being social. One can intuitively see how this is the case when considering how one learns how to play a musical instrument. If I study from the same music teacher, the same number of hours, the exact same instrument, under all the same conditions as my friend, the two of us would still not play music in the same way, with the same musical style, etc. Learning is personal.
A problem with class sizes of 25 – 30 is the peer group is often too small to be functional. Not everyone is ready to give feedback when a learner needs it. Larger group sizes are needed for peer review to work. From our experience groups of around 50 – 60 students should be considered as a minimum, groups of 90 – 120 or more is even better.
This really depends on how one defines a group. Since "virtual learning enviroments" is being considered, I would argue that we are really looking at a network as opposed to a group whereby peers become not only the classmates within the same school, but peers that extend beyond the classroom. Regardless, it's the type of interaction between the students that counts, and not the number of students that make up the learner's network.
Teachers (and schools) have the obligation to find innovative ways to connect students and experts in ways that bring about multiple perspectives. Teaching a class is not a mistake, teaching a finite group is.