In the movie Alien (1979), the first alien we see is the facehugger. It is only the size of a baseball mitt.
A few scenes later, the offspring bursts out of a crewmember's chest. (Thanks to Johnny Bones for reminding me of that in his answer.) It is the size of a adult human's arm.
When we see the alien next, a few hours later, it has grown larger than the average human.
Is there an in-universe explanation how the alien gained so much mass? Did it eat food that was aboard the ship?