Imagine that you live in the 40th century and you are eager to fight on the battlefield. Well, fellow Human, here is where you will stand in the food chain, and why size clearly matters, with a Warhammer 40K size comparison:

On the left there is a cute little human girl.
Next to her, a basic human (well, ultra buffed chaos cultist, not a random dude).
After that, a tyranid Termagaunt: less tall but way longer and beefier.
Then it’s a drawn between an Ultramarine Space Marines Infernus and a Chaos Daemon Bloodletter. The daemon is bent in too so it looks a little smaller, but he’s probably a little taller instead.
After that, a Terminator Space Marines.
Then a Captain in Terminator Armour: in fact he’s the same size as a standard Terminator, but with more decorations and he’s standing on a stone…
Then a Tyranid Psychophage: you don’t want to face this one. No you don’t.
Except if you are the last one (my biggest one at the moment, but by no mean the biggest Warhammer 40K miniature): a space marines Redemptor Dreadnought.
In a nutshell: if you are a random human being, do not try to fight in the 40th century. 🙂