Size is power
Characters and such grow in size when they become more powerful, or more powerful or weaker variants of certain NPC, enemy, monster, etc. is sized accordingly.
Alternate name: Size-coded NPCs
The first video game about Size is power was released in 1999.
So, human children being weaker than human adults does NOT count. Regular humans being weaker than some giant humanoids does not count either. However, if you see a person who is 5'7" and another who is 5'11" and the latter is inexplicably more powerful (well, size does give additional strength same way being fat does), then this would count.
Some 3D games take this literally with the characters growing in size with each level or powerup.
Makes absolutely no sense when talking about skill or other non-physical power.