Either Arclight Vel-Koz is invading the land of Lilliputian in his display picture, or he's bigger than the Baron Nashor, when compared to the man he's incinerating. Seems his picture art doesn't match the game sizes (which is fine).
Helmet bro is a tiny man
with a big heart
RIP
See: Malphite
Velkoz is larger than he is shown in-game. Same with a lot of champions: 
Vel'koz seems to be overall a bit smaller than he is in-universe(Just look at the first-contact-video)
However some skins depict champions in giant form,but leave the size of their ingame model unchanged. I mean look at BC-Vel'koz,the dude is ripping through a skyscraper with one of his tentacles(so yeah he is huge) or the Mecha skins,who depict Aatrox,Malphite(and probably Kha'Zix) as giant mecha
The interesting thing about Vel'Koz is his lore depicts him entering Zilean's tower and investigating things inside it. How did he do this if he was the size he is constantly depicted as?
