@Galaxy:
Zoro increasing the size of his arms.
One Piece is still a Shonen manga at heart, so "generic" Shonen things like that still exist.
I dont deny cliches of some sorts but at same time Oda introduced us to life return techniques. He is also introducing us to different styles of fighting.
Though they seem to have some common elements like Sai mastering his family fighting style and "unlocking" its strongest move kind of same as when some Rokushiki master gets all 6 paths perfected and can start using Rokuougun.
What I want to say that life return Rokushiki skill and Lao G muscle restoring skill can be from "same family".
For what we know Lao G teached Law matrial arts, Vergo and Lao G were in same crew for quite some time.
Later Vergo was sent on mission to Marines where he was very likely thought Rokushiki and its very likely he gotten introduced to life return techniques.
Vergos growing bigger without any sort of explanation left some space for interpretations and thats what I am doing.
What I wanted to say is that Vergo might have used variation of skill that Lao G used to grow big. He doesnt have to store muscles for decades like Lao G but perhaps he keeps it as reserve so he can grow bigger.
Technique of life return made Lucci slim down as Robzilla said and then get back to his muscled form. So its likely that Vergo used skill "from same family" of body manipulation techniques that masters of matrial arts in OP words seem to be capable of using. Maybe even Zoros muscle growth thing is connected to him training and some bits of skills comming out of training in natural way like for example Sanjis Sky Walk being developed from running but being same thing that Rokushiki masters get from their training on that specified skill.
I dont know if I worded it properly but I do hope what I wanted to say is more clear now.