You need to examine the combat in the atmosphere processor:-
The first of the Marines taken out were Deitrich (flamethrower), Wiezbowski (flamethrower) and Apone (flamethrower). Frost was a lucky bonus (accidentally torched) but anyway was armed with a pistol, as was Crowe (exploding ammo bag), who would have been armed with an (empty) pulse-rifle.
That left Hudson (empty pulse-rifle), Hicks (shotgun), Drake (smartgun), Vasquez (smartgun). It does however indicate the intelligence of the aliens, as at this point the only weapons that had been fired were the incinerators, and those were the Marines they took out first.
So, the initial ambush had taken out over 50% of the platoon without a single alien warrior being killed. But despite being heavily outnumbered, they were able to fight their way out, losing only Drake in the fall-back. It's highly probable that after they torched the chest-burster, had Gorman ordered the pull-back the same time Ripley had suggested it, most if not all of the Marines would have made it.
By the time of the battle in operations, they have no flamethrowers, only a few grenades. We have Hicks, Hudson, Vasquez and Ripley armed with a pulse-rifle and about 40 rounds each (Hudson and Vasquez have even less than this as they took out the facehuggers in medlab), and Gorman with a pistol. In this case however, extra ammo wouldn't have made much of a difference, they were all still firing when Hudson was taken, but Vasquez and Gorman may have survived the retreat through the vents. Of course, had Gorman ordered the pull-back as suggested earlier in the Atmosphere Processor, there may have been at least twice as many Marines in operations as there were.
So in Aliens the deck was stacked against the Marines from the outset.
The United Systems Military in Resurrection are of course a bunch of incompetant boobs. They're breeding alien creatures with acid for blood and decide to do it on a space-ship? What, no secret asteroid bases anywhere?
The moment things go south, Perez orders an evacuation rather than a defence. Ironic, that Perez screwed up by ordering a retreat, but Gorman screwed up by not doing so!
Hell, they even allowed three of the mercs to sneak aboard with weapons! The chair thing I can understand, but not scanning the sleeves or checking the flask properly?