What the hell did I gain? My quads got huge (from all the hiking with a 50# pack, I assume), I went from a 34 to a 39 chest, and I would submit that all ancillary not-usually-measured parts got significantly bigger. For other measurables, I went from doing 6 pullups, 60 situps and a 20min 3-mile to 17 pullups, 120 situps and an 18-min 3 mile in those same 3 months. And I promise the drill instructors did not let me use chemical "help".
"Most professional bodybuilders", I would submit, are a) not 17-19 years old, b) not still growing, c) not in a basic training environment and d) are at a higher weight floor. But what do I know? Except that it IS physically possible to put on 30 pounds of "not fat" in 13 weeks. And I'd submit that Vonleh is probably closer to where I was in boot camp (skinny, athletic kid who hasn't filled out yet) than to a professional bodybuilder.