As I recall, the original Alien was pretty tall. I'm guessing around the 7-foot range. Considering the performer's face was in the creature's neck and the Alien head was on top of that, I'd say it might even have been taller. (I'll have to dig through my olde Starlog magazines for pics.)
The Aliens in the second movie were so abundant, "normal" sized actors were used. I attribute the difference in sizes due to the hosts; if the Alien in the first movie was a result of the huge Space Jockey, naturally it would be bigger. Likewise offspring of human hosts would take on more human characteristics, including height.
The only PURE Alien might be the Queen, but even she probably underwent some creative license here and there.
Let's just say that these figures ROCK next to Hasbro's Star Wars line.
And speaking of the Space Jockey, wasn't a statue made of that thing? I wonder if it was close to 1:18 scale.