My theory on why the Aliens are skinny, and have such skeletally thin limbs, yet are very strong.
The Aliens do have an endoskeleton, and what has been called "exoskeleton" is really chitin armor.
I compare the hide of the Alien to the IBA "interceptor" armor: The hard parts need armor piercing rounds to have any hope of penetrating, while the softer parts are merely good enough for protecting against shrapnel and are not as good at dealing with munitions.
None the less, the Aliens, all of their stages from hugger to Queen have an endoskeleton.
Bones are moved by skeletal muscles.
On the Aliens the bones and muscles may well be made of different substance, but function in a similar way. The muscles still move the bones by pulling them from insertion point to corresponding insertion point, and contract in order to move the bone in the same direction as that muscle is pulling.
Tendons are more dense than muscle, and have a similar tensile strength, but are smaller in diameter, and are mostly used to connect the muscle to the bone at the point of insertion.
The Alien's muscles structure may be more similar to tendon structure, and function like a contracting steel cable, very strong, yet without a huge diameter needed for the tensile strength.
So human muscles are more like a fiber-sponge, and the Aliens are more like a steel cable.
That would explain why their limbs have relative thin circumference, yet surprisingly strong.
Predators on the other hand, would function much more like a humans. Their muscles, while stronger than ours, still are more like the fiber-sponge with the ability to engorge with blood and flex, and have a larger circumference compared to their tensile strength than an Alien of equal tensile strength would have.
Basically if there where an Alien that could Bench Press 1 ton, and a Pred that could also BP one ton, the Pred's arms would still be at least twice the circumference as the Alien's, but both would be equal in tensile strength.
