I am a big fan of Marvel comics. In the Hulk franchise there is a character known as the Leader (Samuel Sterns). This bad guy got bombarded with gamma radiation, which resulted in a super-sized brain and superior intelligence. But is it biologically possible to live with a brain of this size?
When we look at the human fossil record, it is clear that there is an increase in brain size. The early hominids, such as Australopithecus and Paranthropus, have a cranial capacity of about 400 to 500 cc. The first members of our own genus Homo show a quick increase in brain size from 600 (H. habilis) to 800 cc (H. ergaster). Modern humans are endowed with a cranial volume of 1200 cc on average. Men have a slightly higher capacity compared to women, but I won't go into this (precarious) issue.
Is the Leader unrealistic? No, because he obviously extracts the needed energy input for his enormous brain from the gamma radiation! So...says...the Leader!