It's rampant in professional sports that athletes are taking something, at the very least in the offseason. You'd be nuts to think a player being paid millions isn't looking to utilize using a chemical advantage, especially given their access to these boutique compounds that are available. These guys aren't taking Dbol and Tren
For Petey, 13lbs isn't outlandish. I doubt he's put on 13lbs of pure muscle, almost guaranteed a portion of that weight is fat/water. For someone of his height and frame, and likely minimal heavy weight training in his career prior, 13lbs would be pretty easy to pack on in 3 months. Newbie gains are always great.
It's not like he's Aaron Donald, a guy who is maxed out for his frame, and where putting on 13lbs of muscle would be pretty insane.