It's just from what I've seen personally. I don't think there's enough evidence to prove what ingredients may or may not cause tear staining. Like I said, Jackson got terrible eye boogers on Fromm and Wellness, both of which are high quality foods, so maybe there was one little ingredient in there that bothered him. MAYBE it wasn't the food at all, but just environmental and it being a coincidence I just happened to be feeding it or something. Also, I think small dogs in general are more prone to them (as I think you said) so comparing big dogs vs. little dogs probably isn't a good comparison. Sometimes I think it's just genetics over food or environment.
I'm a dogsitter/walker and basically all my relatives have dogs too. I only know three dogs who eat "high quality" foods (A Rottie on TOTW, a Lab on Natural Balance, and a Beagle mix on Orijen). Maybe by pure coincidence, but those 3 dogs are in GREAT physical shape, their energy is awesome, their coats are sooo shiny and soft (it's especially noticeable on the Rottie and the Lab), and just in fantastic shape in general. Compared to, say, my aunts Rottweiler (who was 3yrs old when she rehomed him) who had dandruff galore, his skin was terrible, his breath/teeth were horrible, he always had ear infections, he was constantly itching/scratching, and was on "grocery store brands" his whole life. My grandma's Lab is the same way... she seems healthy, to a normal person who doesn't necessarily pay attention to dogs like I do, LOL, but I see an overweight dog, one who itches constantly, terrible gross teeth, dull coat, shedding like crazy, etc. All looked at as "normal dog stuff" by most. Oh, not to mention, they all poop a TON. I watch 2 Boxers on Science Diet, and I have never in my life seen dogs that poop as much as they do, and as big and smelly. SD-fed dogs in my experience all have this problem. The Rottie on TOTW has poops that aren't all that much bigger than Jackson's and they're easy to pick up.
*shrugs* I don't know... who knows really! Sometimes I second guess myself too. Is it really worth it? Who
really knows. There hasn't been any good studies proving one either which way. I mean, I've been living off terrible foods for most of my life (pizza, fast food, sodas, etc) and I'm surprisingly healthy besides being a bit overweight. But I've recently made a huge change in my diet and I've already noticed my face clearing up, my energy levels are up, I'm more motivated in general, etc. Do I look any different to anyone else on the outside? Probably not. But I know it's healthier for me. Could Jackson survive and live a good life on Purina? Probably. But why NOT do better? If hard times came on, I wouldn't hesitate to feed a food like that. But I can afford more right now, so I might as well, I suppose.
Argh -- I just went on a total tangent and half of it had nothing to do with eye boogers.
I'm being a blabber mouth. Sorry, I've got a lot of opinions, hahaha. I commend anyone who just read all that.