Sorry for the delay, but I find angel to have a compelling argument - in order to grow cows need nutrients. It's not just 'grow fat' or 'heavier', if they simply weighed more because they had 100 pounds of feed stopped up in them, the meat packing industry would be pissed and start buying on the basis of gutted carcass weight or something. In order to get said nutrients they need their gut bacteria to break down their food, otherwise it'd inedible to them. In a sense Cows digest the bacteria, not the plant matter they consume.
If there are 'bucket loads' of scientific studies, it shouldn't be hard to give a reputable source. I'll admit that I haven't studied the issue. I know there's weight gain when growing animals are given antibiotics. I know they'll maintain weight/growth if given less food along with antibiotics. Why? That's trickier.
For example, this report shouldn't be taken to heart because it's by a student, not curated or peer reviewed, but it's at least simple and lists more references. It says that the growth isn't because food processing is being disrupted, but because the animal isn't spending resources developing immune responses it otherwise would and that most of the bacteria effected are in the large intestine, which provides minimal nutrition extraction as opposed to the bacteria in the stomach and small intestine.
Also, 16% more growth on 7% less feed is significant, which is why a lot of pig farmers today are perfectly willing to give up on routine antibiotic regimes for pigs in other stages where it's much less effective, but want to keep it during the starter/weanling stage.