There's a perfectly valid, consistent explanation given by the films:
Aliens collect traits as they go. Scott suggested it back in the 70s.
So Big Chap is Alien - Jockey - Human.
The Runner is Alien - Jockey - Human - Dog, and you can see the biomechanical details are taking a hit.
Alien Resurrection is awash with human. AvP and AvPR, there'd be no Jockey in the mix either, so they look similar.
Covenant even bares this out with the new Alien being fleshy and largely devoid of biomechanical detailing, as Engineer/Jockey hasn't been added to the mix yet.
Aliens have a basic shape but the detailing is a product of intergenerational trait collection. Each generation builds on the last, they don't reset.