I really do think it is a size thing and not a cultural thing. There has been some scary and interesting news in The U.S. about things that happen when people gather in large groups. I don't think they would say or do some of those things if they were alone or in smaller groups. Being a part of a big group makes them feel powerful and invincible- and can lead to irrational, uncivilized thoughts and actions.
There is some interesting science behind "Mob Mentality" or "Herd Mentality" or "Crowd Psychology" you can read about. Gustav LeBon said, “In a crowd … a man descends several rungs in the ladder of civilization. Isolated, he may be a cultivated individual; in a crowd, he is a barbarian.” That phrase is just as true today as it was in the 19th century. People in big groups can be scary.
What prevents Disney from asking tour groups to split up and not travel in more than x number of individuals? Seems it would be safer and more pleasant for everyone.
I've seen lots of large groups at WDW, groups from America as well as other countries, though none compare to the obnoxious behavior of the Brazilian Tour Groups.