I’m watching the original Alien on TV. When the creature bursts out of its host’s chest, it’s no larger than a cat. Within a few hours – before it makes its next kill – it’s larger than a human. (This pattern is repeated in all the sequels.) How? Nothing can come of nothing, and there’s no hint that the Alien has been raiding the ship’s food stores. Even supposing it’s natural for it to grow that fast, where did it acquire all that biomass?
Duh, it has acid for blood and acid’s, you know, good for you.
Yeah, I often wondered about that too, what do they eat for example that could make them grow so big? They seem to use us tasty protein rich beings as mere incubators.
A plot hole that the enjoyability of the film fills in nicely 
Well, if they have acid for blood, maybe it ate the ship’s batteries?
Well, considering the alien has a kind of ‘exoskeleton’, my best guess is that when it molts, it puffs itself up like crabs and lobsters do; maybe molts every hour, puffing up more each time, just growing really really fast.
Perhaps they have a really high metabolism.
Yeah – but it still couldn’t get any bigger (i.e., more massive) with each molting unless it ate something.
The only thing I can think of is that perhaps it raided the ship’s pantry. And yes, I realize, there’s no evidence of this in the film, but I can’t think of anything else.
I blame creative license.
Better still: “Wizard did it.”
I think it simply found something to eat. For the purpose of storytelling, it would lessened the “holy shit it’s big” surprise a bit if they showed it eating.
Simpsons, right?
From one of the “Treehouse of Terror” Halloween specials – “Stretch Dude and Clobber-Girl,” guest-starring Lucy Lawless, who uses that to explain to a Xena nerdfan how her horse changed color between one scene and the next. Then a costumed Comic Book Guy mounts a helicopter attack and snatches Lucy.
LL: I’m not Xena, I’m Lucy Lawless!
CBG: Please, I’m not crazy! I just want to take you back to my lair and make you my bride!
(Hey, who wouldn’t? 
)
Well, I’ll just point out that mass can be drawn from anywhere, it doesn’t have to be from absorbing solid objects.
Though, in a spaceship environment, the absorbtion of that much mass worth of air, for example, would probably have been noticed on some key pressure gauge somewhere. I’ve always like the idea of creatures who are capable of growing or shrinking very quickly simply by venting or absorbing air and water.
In Alan Dean Foster’s novelization, there is a brief scene where the crew finds a bunch of cans that had been chewed through. Small addition, but a nice touch.
Also, the thing has metal teeth. Maybe it needs to/can eat metal or plastic?
Alternative geek explanation…
Maybe the chest burster was incredibly DENSE. So when it started growing it was capable of trading density for size.
It’s got acid for blood an super strong teeth. My guess? It ate a wall.
I remember! And then he took “the one working phases ever built” out of its original packaging and ruined its value! hahaha.
Doh. that should be “only working phaser”
Then it would have weighed too much for its host to be so oblivious to its presence, prior to the chest-bursting scene.
There was also the sequel (#3 or 4, I don’t remember which) when the alien burst out of a dog and then extended itself to show that it was three times the size of the dog it was growing in. :dubious:
I blame Alien (and maybe The Thing) for the horde of metamorphic, physics-defying creepy crawlers that have infested the movie screen for the past 20 years or so. Nowadays, it seems like * every * voracious Evil From Beyond has the ability to grow enormously overnight. For example, the movie Species did the whole “age 15 years overnight fueled by a handful of Fritos” thing. Not to mention the almost universal “slice off an alien’s body part and it reweaves it from a bunch of CGI tendrils.”
One of my least favorite movie cliches is shape changing that completely ignores physics, e.g. The Faculty in which an alien manifested as a teenage girl morphs into a beast with the size and mass of a John Deere tractor.