To be honest, I have no idea. User shabble has some ideas (in a sibling comment to this one), but reading that gives me the impression that this is a little bi

To be honest, I have no idea. User shabble has some ideas (in a sibling comment to this one), but reading that gives me the impression that this is a little bit of a theoretical concept. It's not. It's very widely practiced, with the explicit intent of "promoting growth". E.g., a search like [1] calls up a large number of references to it. [1] https://www.google.com/search?q=how+do+antibiotics+fatten+animals https://www.google.com/search?q=how+do+antibiotics+fatten+an...