Why do Aliens have such large heads?

[Xhan]

Why is the Queen so large? Surely inferior mobility and being an easier target are undesirable aspects? Why does it need a voodoo mask? Let's do science.

Because you can't have a stolen from anime vagina power boss fight is the Queen is able to slip around the powerloader and f**k Ripley's shit up by ripping the back of her head off, which a normal Alien would have done. The Queen looks the way she does for convention and aesthetics, same reason a Pulse Rifle ammo counter only works for left handed people.

[Mus]

Why is the Queen so large?

Gotta have somewhere for all them eggs before they pass into the ovipositor.

It could just have a very large stomach. Now that's sexy.

[OpenMaw]

same reason a Pulse Rifle ammo counter only works for left handed people.

and requires you to stop firing to check. 

[Sharp Sticks]

She could neither see nor hear it, but she could feel its presence. Now she cursed light and sound. The eyeless face cared nothing for such specialized perceptions. It used the old senses: vibration, electricity, fear.

-Aliens: Hive

Oh, and its a giant penis.

[Kriszilla]

I wouldn't mind it being to do with psychic abilities tbh, I mean, how does the alien always seem to know where the people are, even in really big areas like the colony, and the prison?

Alternatively it could be a store for powerful sensory organs, or just for the muscles for the inner jaw 'launching' action.

[Crayola Ola]

It's head is that big because Giger wanted it to look that way, there is no meaning to it. Simple.

[Inverse Effect]

That is one deadly penis...

Yeah, it bites back.

[Valaquen]

It's head is that big because Giger wanted it to look that way, there is no meaning to it. Simple.

Actually...

'During the last shot you can see the Alien's "tongue" slowly coming out. And his teeth came because I prefer always to have these big long heads for the monster, because I worked as an industrial designer. Every object needs to have a function. So if it has a long head there's space for a long "tongue".'
HR Giger.

[SpaceMarines]

That, and Giger has an obsession with phallic shapes.

[chupacabras acheronsis]

that aint a valid explanation within the canon of the movies.

[SM]

Jockies have an obsession with phallic shapes.

Just look at the gigantic cock the dead one is sitting under.

[chupacabras acheronsis]

We dont even know if their penises look like ours at all... if they have ones.

[SM]

Doesn't change the shape from being phallic even if it has nothing to do with schlongs in the wonderful world of Jockies.