Brain capacity of Cro-magnon man vs Modern man
Some of the possible explanations are summarized in Selection for smaller brains in Holocene human evolution. It's a blog post (but from a well-regarded anthropologist and heavily referenced), but I'll excerpt a few quotes here:


  Less well known is that the terminal Pleistocene and Holocene (ca. 30,000 years ago to present) witnessed a substantial decline in endocranial volume ... This decrease of approximately 240 ml in 10,000 years is nearly 36 times the rate of increase during the previous 800,000 years.  ... 


He considered one possible explanation, that the smaller brain size is due to smaller stature, and determined that it's not likely:


  Here, I apply a quantitative genetic model to test the hypothesis that Holocene evolution of brain size may be explained by reductions in body size. ... The net change in endocranial volume, roughly 130 ml from the terminal Pleistocene to late Holocene skeletal sample would predict a reduction in stature of 13 cm, if the brain size had changed only because of correlated changes in stature. That degree of stature reduction is not biologically impossible although it would be extreme. ... The evidence suggests substantial reductions in brain size in some recent human populations, more than can be explained by correlated changes in body size. 


He then lists six possible alternate explanations, which are not mutually exclusive:


Chance. 
Plasticity.
Climate. 
Nutrition. 
Function.
Development. 


He spends some time discussing each but concludes that at the present time the cause can't be definitely determined. 

Here's his conclusion:


  The available skeletal samples show a reduction in endocranial volume or vault dimensions ... This reduction cannot be explained as an allometric consequence of reductions of body mass or stature in these populations. The large population numbers in these Holocene populations, particularly in post-agricultural Europe and China, rule out genetic drift as an explanation for smaller endocranial volume. ... Therefore, smaller endocranial volume was correlated with higher fitness during the recent evolution of these populations. Several hypotheses may explain the reduction of brain size in Holocene populations, and further work will be necessary to uncover the developmental and functional consequences of smaller brains.