I've watched a lot of UK/Eu videos of perch fishing and I think there are a few other differences.... I just saw a picture of the new state record Yellow Perch for Pennsylvania, it was just shy of 16" and weighed 2lb14. It looked like a huge fat belly with a perch top side... I know occasionally you guys can get perch to 5lb. My thought is in europe perch fill a habitat niche which here in North America is filled by smallmouth bass in similar types of waters. You have Zander, we have Walleye which are quite similar but not identical, even your pike have some "average" differences to look at compared to ours. All the same, all a little different.
I agree - really large perch is foung in the waters with low competition with similar species. This is the example of a perch stocked into a lake with the fauna of only salmonids and minnows - here it is top predator, weight up to 8 pounds. Location: Uega Lake up the Okhota River, 200 km from Okhotsk in the Russian Far East.
100 years ago the yellow persh was stocked into the Shilka River (source of the Amur River). Till now the species is totally absent in the lower reaches of the Amur - the river has diverse fish fauna with numerous predators which occupy all possible niches...