2 Answers

Socially acceptable?   I'm not really sure what you mean....
About six months ago, my sister and I were in San Francisco and went to visit the Japanese Gardens.  There were people from all over the world.  We could hear rvarious languages being spoken everywhere, as well as accented English.  It was rather obvious who was an American and who was visiting from outside the USA.
The most interesting thing we noticed was that the American tourists were, with little or no exception, noticeably more chunky that those from outside the US. 
This doesn't answer the question, but I'll bet Americans are more accustomed to seeing chunky people, and way more likely to be one themselves. If we weren't, en masse (pun intended) , tolerant of it, there would be a civil uprising and crusade to discredit chunky persons. 
I am actually in a personal battle of the bulge.  Don't want to be chunky. And I am one of the most congenial, least argumentative persons I come across on a daily basis.
 

There are alot of chunky people here in the UK as well as the rest of the world. Must say, I find most people who are a bit chunky happier than most.