This idea seems counterintuitive and is certainly not how schools are marketed--the small the size, the better the instruction--more personalized. That is the norm, right?
Here is the deal, though. I think Gladwell (and score of other researchers) are right.
My best year-long experience in a class has been in a class of about 15. My worst was in a class of six. My larger classes tend to be a more mixed bag: male:female, strong:middling:weak students, learning styles and differences. We do a lot of collaboration and discussion in my class and we need a diverse group for the best work to be done.
The small class I mentioned was all male, all high achieving and not particularly racially diverse. I was so excited for the small class but left many days feeling exhausted by it. The discussions were mediocre and the group work felt 'lowest common denominator.' They all earned good grades but I didn't feel as if I challenged them to be better students or citizens. This experience was early in my career and I know I would do things differently now. But...
Part of the problem was size. Sometimes students need a place to just be. They want to be seen and heard but not front and center all the time. This is true of both introverted and extroverted students. Sometimes you just want to be let be; to let someone else do the heavy lifting for a minute. Sometimes you need time to process what you are hearing and thinking. Sometimes you need white space to think through a challenging statement; a safe place to wrestle with a new idea. I honestly believe adding even a few more students would have been helpful.