As humans get bigger over the past many decades (not just fatter, but taller, etc.), are brains getting noticably bigger, and if so, is it useable matter or just “empty space”? :dubious:
Just to address the real question, there is no known correlation between brain size and intellect in modern human populations. Yao Ming has a much bigger brain than Paul Hamm, but that doesn’t make Yao Ming any smarter.
Our brain power probably comes as much from the internal wiring (number of connexctions between neurons) as anything else. If you could measure THAT, you might find a correlation between it and some aspects of intelligence.
I’ve seen no stats either way, but it would be difficult for human brains to get significantly bigger. Babies’ heads barely fit through the birth canal as it is, which is why human births are so traumatic compared to other mammals’ births.
Women’s pelvises would have to get wider, which in turn would mean they could hardly walk properly let alone run. So it’s hard to see that it’s possible.
We might also consider that nervous impulses don’t move all that quickly (on the order of meters/second). How big can our brains get before speed becomes a limiting problem? We can be super-smart, or quick, but not both.
We must consider the possibility that human heads are getting larger, as we seek to mate with smart people and a correlation between smartness and head size is perceived.
As a result, in four generations tops, pregnant women will explode in their eighth month.
Four generations? Eighth month? Heck, my wife exploded at me starting in her first month during each of her three pregnancies 
To say that brain size isn’t a determining factor or indicator of intelligence isn’t exactly a true statement either. The surface area of the brain is though. The number of sulci (the grooves and ridges) in the brain increase the “surface area”. If the human brain were smoother (as some mammals are) the intelligence level would reflect it.
The differences in size among human brains is relatively insignificant and not generally accepted as a determining factor in intelligence.
There are several physical and environmental reasons that can be attributed to a persons intelligence.
Somewhat related to the OP is correlational evidence that indicates the human brain has evolved and an increase in gyri, fissures and/or sulci is the result.
Thus humans are “more intelligent” than our ancestors because of an increase in the surface area of the brain.
cortical folding-The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortex—about 1.5 m2 (16 ft2) in an adult—to fit within the cranium
I remember reading somewhere way back that brain size in a species is a decent indicator for intelligence, but not for individuals. Sounds reasonable to me.
Perhaps a tendency to greater post-birth brain development would deal with this.
When comparing species, anthropolgists occasionally use the encephalization quotient, which is related to the ratio of brain weight to body weight.