We raise chickens and I can state firmly that antibiotic use DOES increase size. For example, this August 30, three farms, including ours shared a batch a broiler chicks. So we have the "control" of the same chicks, same date, from the same hatchery. The other two farms used medicated feed for their first several weeks of feeding. We did not. The other people chickens became monster size at 7-8 weeks, unable to walk. Our chickens grew much more slowly, and we butchered them at about 12 weeks at nice plump 6-7 pound birds.
Raising animals correctly does take longer and costs more, but in the long run, we feel better about what we are putting on the table.