Other mistake: In the scene where Ripley throws the basketball at Johner, Christie says to Ripley, "Ok, I've got a new game, tag .." He hits her with the handles straight in the face once, he misses the next shot, misses again, and Ripley hits him to the ground, but you neither see or hear the handles fall. (00:23:15)
Other mistake: At the end of the movie, as the ship is approaching Earth, the Earth is backwards. Africa is obviously pointing in the wrong direction, as if the image has been reversed.
Other mistake: When Ripley pulls the knife out of her hand the blade is smoking as her acid blood is melting the steel, however shortly after she touches the girls face and hands, and the acid blood has no effect.
Other mistake: The remaining crew of the Betty, plus 2 others, have to swim 90 feet underwater. This they do, but once they are out of the water, the only one who is wet is Winona Ryder.
Chosen answer: Well firstly the queen was probably genetically engineered, like Ripley herself. A few days might be all the time they need to have a fully grown queen created. Secondly the 8 incubated victims were only the latest batch, they had been incubating people with xenomorphs for quite a time I suspect.
lionhead
With regards to the quick growth of the alien queen, it is standard for the xenomorph in nearly every film they appear (Aliens is the only exception, the only chestburster shown in the film is quickly killed by Apone) to grow to full size in around one day. Presumably the same is true for the queen.
BaconIsMyBFF