Actually, the energy theory it likely incorrect. The more likely candidate for the connection between antibiotics and weight gain is the disruption they cause to the microbiome. This causes a metabolic disruption that impedes the breakdown of starches in the gut, and causes them to be converted to fat.

This is not just in cattle, by the way. It's also in humans. Antibiotic over-prescription has been linked to obesity and fatty liver disease. See, for example: https://www.nature.com/articles/pr2014157