When we see them in swarms they're the size of the average bat..up close they're the size of velociraptors with teeth and claws to boot.
Compare the scenes where Carolyn gets stuck exploring the cave, when Shazza falls prey to them - and later when we see them inside the cargo hold and in the rain towards the end.
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They made it rather clear that those were two different predatory species. You aren't supposed to believe that the bats are the same huge creatures you see in other parts of the film. The entire planet isn't populated by one single species, but by several nocturnal species. The bats are likely the primary reason all day-walking life was extinguished on the planet; more so than the larger creatures.
shareThere's another interesting question as to the origin of the dinosaur-like animals whose bones are shown after the crashlanding. If the eclipse is as periodic as it is made out to be those bones seem out of place.
shareWhat I don't understand is why the two main species we see - the bats and velocoraptors - are both nocturnal. Where are all the non-nocturnal species? The exclipse only happens once every 22 years, so there should be alot of other species filling in while the world is light. It can't be because the atmosphere was too harsh - the humans survived it after all.
shareThey are up in the air and further away, of course they look smaller, ya dingus.
shareNothing about the aliens adds up. You're right with the size (the first time we saw them swarming like bats, I literally thought it was a second, different species than the kind that attacked the first guy in the cave), but also with their ability to find things with echolocation, whether or not they are attracted to movement or hurt by light, how they can survive by stripping the whole planet of resources to the point of cannibalism... on and on. Heck, there's the one scene where they're moving through the dark and the alien beasties are swarming just outside of the shield of illumination, but when the light goes out for a full minute (or more), they have time to light up a bunch of torches without getting massacred in six seconds?
But, Riddick also magically knows they're attracted to blood and that lying still in the bottom of a trench, or doing a little face-to-face dance, will make them not see you and attack you. He knows a lot about them for some reason (no reason at all).