The brain does not get heavier when it gains more knowledge. They took Albert Einstein's brain out following his death and studies since have shown that the weight and size of his brain were about the same as the average man.
There were only two noticeable features of his brain that were unusual. Firstly, a region of the brain called the inferior parietal region was wider than normal. This showed that he had additional mathematical reasoning powers over that of Joe Public. Secondly, a brain fissure called the sulcus ended abruptly. The significance of this remains controversial as only some of the functions of the sulcus are known for certain.
I've been privileged to see two preserved portions of his brain at separate american universities and believe me, they look exactly like other brains I've seen.
Finally, let's use some reverse logic. PM (Autopsy) examinations of dwarfs, pygmies and other races of small stature have been performed for centuries, often for macabre purposes. In a small person, the brain is always found to be smaller and lighter than normal because everything remains more or less in proportion.
Dwarfs and people of small stature are as capable of absorbing knowledge and acheiving high intelligence just like the rest of us. The fact that their brains are smaller and lighter, doesn't make them imbeciles.