Having a brother who suffers epilepsy and a couple of other shit side effects after a bad cranial trauma, I made a lot of research about parkinson, memory lapse and epilepsy caused by shots to the head and though I am not a rocket in science and neither a neuro surgeon, from what I understood across my multiples researches, the fact to get knocked out doesn't play a major role in the whole thing; what is bad are the micro tearing (or major ones if you're very unlucky on a specific occasions) that happens in your brain after repeated shots. Knocked out or not, these micro tearing happens and at some point, consequences start to point out.
For example, probably that most of Ali problems come from his sanguinary fights with Smoking Joe, he wasn't necessarily knocked out or down but the beating his brain took during these sole 3 fights is tremendous. The more you take hit to the head, the more you get a candidate for parkison and mid-long time problems. When you have a bull strategy and no defense like Hatton, Andrade or Gatti, you're an even more susceptible candidate to these things, the weight at which you fight surely can play a role too as it increases the power of the shots you're absorbing and then the damage your brain absorb.