He didn't specify that the bigger kingdom were all equal size, nor would that usually be the case as it's pretty inefficient. Once you're at max protection, grabs against you are around -80% and losses are significantly lower. Add KD NW protection to that and you're looking at an 8 to 1 net difference in gains easily. Also add that the loner can hit a new province each grab and thereby avoid protection penalties altogether. It's been proven time and time again that the system can easily be abused, look around on the forum as it's posted about regularly with concrete examples.
I don't actually agree that the system should be removed. I think it's good to encourage direct competition between kingdoms of similar size. The current system just isn't very effective at that. I'd be in favour of slightly harsher penalties for hitting into smaller kingdoms than now, combined with penalties for hitting into larger kingdoms as well. That would directly harm the loner in the above scenario (due to how GB protection works) and would actively discourage safe bottomfeeding by lowering gains.