I didn't read a lot of analysises on difference between Alien and Aliens, and what makes Cameron's film to look more action movie in comparison to the first one, but my personal explanation for this is because Aliens has bigger variety of places when events happen (Narcissus, Gateway's medical room, conference room, Ripley's cubicle, Sulaco... APC... LV-426...) and characters on screen are always busy with something, whether it is work or friendly chatter. Thus, the density of events/persons for footage is higher, making the film to seem more action-filled, than its ancestor, with no bigger number of violent scenes here. THe simple way I distinguish it.