the alien(Ceph) in crysis1 and crysis2 are not that different at all!

The point is: all the enemeis you encounter in crysis1 (except in "the core") are not mechas, but drones. And only those you fight in "the core" are actual life forms.

This also explains why all enemies(again, except in "the core") in crysis1 self-destructs when disabled( which is a very reasonable "function" for drones), while no enemy in c2 self-destructs, because they're actually soilders with powerd armor.

And if you compare those "naked aliens" in "the core" to the ceph grunts in c2, they're actually very similar, both in physical form(if you strip away their armors in c2) and means of attack.

The "naked alien" in "the core" have 2 sub-classes: ranged and melee, while the basic ceph soilders in c2 are also divided into 2 sub-class: ranged and melee.

The theory could be that the ceph force in c1 is either vastly under-manned, or maybe they're just not that "head-on firefight" type (think of someting like an engineer/support class). So their primary way of combat is to build massive "war robots"(those common enemies you encounter throughout the game), and let the drones do the heavy lifting. And this also explains why most of "the core" resembles factory areas.

The ceph force in c2, on the other hand, is either well manned or more of this "head-on firefight" type (something like a soilder/assualt class), so they are willing to put actual soilders in the field.

Very bad and very late attempt at trying to make sense of an unexplained change.

Very bad and very late attempt at trying to make sense of an unexplained change.
late? indeed.

bad? why?

Cause there is no reason in hell a species that can send killer drone squads would attempt to battle with regular infantery.
Plus they apperently gave up on the flying exoskeletons. Plus every last unit, weither drone or exoskeleton had these mechanical tentacles. And for some reason some squishy >squids< decided "you know what? i think these humans have a really neat theme going with their so called 'feet' and stuff" and threw the mech-tentacle menace out of the window so they could pounce u like a humanoid animal, stare into your face and run away so you can shoot em.

Cause there is no reason in hell a species that can send killer drone squads would attempt to battle with regular infantery.
Plus they apperently gave up on the flying exoskeletons. Plus every last unit, weither drone or exoskeleton had these mechanical tentacles. And for some reason some squishy >squids< decided "you know what? i think these humans have a really neat theme going with their so called 'feet' and stuff" and threw the mech-tentacle menace out of the window so they could pounce u like a humanoid animal, stare into your face and run away so you can shoot em.

the hovering drones needed that huge energy buble to power, and they were not as tough as the ceph soilders in c2.(both on supersoilder difficulty)

Agree, aliens from Crysis 1 were robots, while aliens from Crysis 2 are, well aliens

i was more scared of the aliens in C1 than C2...

I haven't read the novel which probably fleshes out more of the backstory but my take was this...

The Aliens attacking New York are the same creatures that were in the alien ship in Crysis. Whether or not they are from that ship or new arrivals from space I don't know. The flying ships would be impractical in the narrow streets & subways of New York.