Exactly this. Weight classes exist for a reason. Inertia is a thing, and smaller people have less of it to throw around. A tiny person will get their shit rocked by a big person, even if the big person is only half-assing their punches. A concussion is when the brain bounces off the inside of the skull, and someone in a higher weight class is much more likely to cause that. Look at how her head snaps back when she gets hit. Now imagine if it was someone her own size, and imagine how much less impact it would have.