I've seen it more than once, and the most recent time was my buddy who showed up specifically trying to prove a point. He was untrained. She was very trained. He didn't succeed at making his point.
Having wrangled toddlers, I'm entirely onboard with the idea that a smaller opponent fighting in unexpected ways can be surprisingly difficult to handle even with a clear weight and strength advantage.
Training exercise is not the same as life-or-death conflict. It may also be the bigger person is holding back because they don't want a manslaughter charge.
He signed up for the gym a month later.