I'm no expert, but my assumption for both is more liberal antibiotic use and the quality of the feedstock. Free ranging chickens (in my experience) taste better than battery-farmed ones. I don't think the chicken KFC buys is free-ranging. As for your second point, antibiotics are often used to get chickens fatter than they can get naturally within a very short timeframe. This publication[1] (from 2018) goes into some detail about the entire practice.
[1] https://pubmed.ncbi.nlm.nih.gov/29537307/