8" really is about the right size even for southern and western swimbait users- I also find that anything bigger does not do real well. Hud designer, triple trout maker, etc., all state that the bigger baits (10,12 inch plus) will catch fish, you just may not really see the cost vs. benefit of using anything larger than the 8" baits though.
I will admit that the only time I throw anything bigger is for muskys- however I have never caught a big bass on a musky lure over 10".
Does not make it 100% correct, no, but again, over a 8" and you start to get diminishing returns.
8inchs in my opinion is big enough to first attract a big fish, but not too big to make them turn without eating every time. Additionally, that size is right for typical forage. I am sure an occasional 12-13 inch bass or crappie do get eaten- but the softer younger ones I have always assumed are preferred forage.