Sorry, but the dude talking in that video is ignoring the most important factor. Ki.
"When you grow up, your bones get bigger, skin tougher, muscles bigger"
For normal humans, this is the case, but for one, Gohan is a Saiyan and for two, he was "Superhuman" pretty much at birth. He's an exception to this rule. You can't look at 4 year old Gohan at the beginning of Z and say "More muscles will make him stronger", because his strength is in the latent Ki that he can channel, not his physical strength. Raditz was literally 10x his mass and only twice as strong... because Ki.
It's actually pretty straight forward. (In that universe of course) If you don't train to maintain your Ki, you gradually lose it. Your body becoming bigger & stronger will not mitigate that loss much.
This is pretty much why things with Ginyu happened the way they did. Ginyu only had the ability to tap into 20k of Goku's Ki. This is why Goku referred to it as "Body and Spirit", because the bulk of Goku's power was his ability to tap into massive amounts of Ki.
But the guy in the video keeps repeating that flawed premise over and over. It's quite annoying really.
Gohan didn't train, so his Ki pool shrank and his technique became sloppy. It's simple and makes plenty of sense.