Antibiotics are used in livestock for the specific purpose of causing weight gain in the livestock. The question is to what extent this causes weight gain in humans.
It might be spurious or not — antibiotics are known to cause weight gain, but I don’t know even vaguely enough about chemistry or biology to know if the quantity of antibiotics in either the environment or the human food chain is high enough to have a noticeable impact.
Antibiotics is basically chemotherapy but for bacteria, killing a lot of the good bacteria in your guts causes all kinds of havoc.
I've only had to use it twice in my life, one time as an adult. I then forgot to also take probiotics at the same time, had stomac issues for weeks after the treatment.