This is the reason animals were much larger in the past, even the primate brain has been proven to be bigger in earlier primates.
Not exactly.
The only cranial size that exceeds our own is Neanderthal, and that is 1) not by much, and 2) for only a portion of the crania examined.
None of the other early primates had cranial sizes approaching our own.
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.