About the same when you relate it to average body size.
Remember that a large brain can not be confused with cognitive ability....
The somewhat go together, but it's not like on necessitates the other.
Homo Habilis lived close to 2 million years before both neanderthals and homo sapiens and its cranial capacity is a lot smaller. As expected.
I'm not a paleontologist, so I don't know.
I'ld guess something along the lines of them not having a habbit of living in environments that facilitate fossilization. Humans on the other hand, like settling near shallow waters, rivers, lakes, etc. And they travel / migrate more whereas chimps tend to stay in their habitat. That's my educated guess, which could certainly be wrong.
Perhaps someone like
@KomatiiteBIF can give us a more informed explanation?