All the sexist stuff aside, what if this had been a male instructor at the 4th dan level? Would the bigger 2nd kyu have thrown with as much force if it were a man he was throwing? I submit that there is a subset of meatballs out there who believe that if a woman wants to train, she better be able to take a pounding. This same subset is also heavily peopled by a yet lower tier of meatball who will kiss a male black belts butt a long time before they'd ever consider planting them with a ton of force.
It's fine to say gender is not an issue, and in a perfect world it'd be true. Gender simply matters.
If a male black belt of less substantial size beat the crap out of a brown belt of substantial size who he felt had been brutish, I think most people would have made allowances for his "instructing" a student. Many of the old guys used to do it, many still do, it's just less of a topic of conversation.
It would be just as wrong. But it would have made way less of a "cute" story than one in which a smaller female basically bounces off of a big guy.
I wasn't there, and I don't know either party, but my immediate perception was in fact affected by the sexes of the participants. And I'll bet most other folks' was too.