There's an article in Iowa Farmer called "Hog Production Nearly Unrecognizable" about changes in the pig business in the last 25 years. It describes, in a neutral way, various transformations, including concentration, multi-site production, all-in all-out and so on.
One interesting point that the article doesn't make - but should be obvious to anyone reading between the lines - is that pretty much everyone put their pigs inside: Big farmers, small farmers, etc. Similar transformations have happened in the pig business before in Europe and Asia.
Pig farming has been filled with radical innovations and the resulting "creative destruction", explaining why modern production is "nearly unrecognizable" from what it was only 25 years ago.