There is a difference how xen creatures look in the first half life games and their sequels.

You can see this with the headcrabs since they used to have small eyes, smoother faces and a shell around their claws so I wanted to try making my own design how the might look like today.

Most likely it's just a redesign but It's nice to give a lore reason for something like this so I would say this is due to xen life adapting to earth's environment.

And since the headcrabs are being used as bioweapons by the combine it's possible that they were either bio engineered by the combine or some genes were blocked to prevent them from maturing properly, preventing them from trying to make nests so the combine wouldn't have to deal with the infestation spreading and making the headcrabs only hunt for more victims.

I'll try drawing other half life creatures later.