alt Hacker NewsIndeed. It is really just tension x time under tension within a sensible rep range (probably around 5 - 30 reps or so). Menno on Youtube has a bunch of videos on this, the link below being the latest one.
https://www.youtube.com/watch?v=rmOBmTZARq8
Basically work the muscle harder and get more jacked. It isn't that hard. Full body workouts are also great for this reason: you can hit a muscle more times per week and be fresher when you hit that muscle, so both the tension and time under tension can be higher vs a body part split.
Time under tension is an imperfect measure, it's just less bad than other measures we could use. Sort of like lines of code in software engineering. Given that, saying it's "just TUT" is misleading.
It could turn out to be that the brain is coordinating hypertrophic biochemical cascades in muscles, and TUT is just a fairly reliable method for inducing this.
I was a competitive powerlifter and trained around pro bodybuilders for years, and in my experience, the only commonality between them was the intense all consuming drive to be absolutely monstrous (and they ate a lot). Some would train for 2 hours a day, some would train for 45 minutes 3x a week, some would use high volume in the 50-70% range and others would focus on 70-85%, some were explosive some were slow and steady, really it was all over the map.