@ G. Mello: Gray matter is NOT mainly cell bodies! Pericaria make up less than 3 % in volume in grisea, glial cells about 1 %, the rest is dendrites and axons.

To my knowledge absolute increases in grey matter are only seen in oedema, reversible reduction in volume only in dehydration. Cortical fields can show relative increase by extensive training ("remapping"). This is because existing neurons are recruited for a specific task, not because of new neurons.

The firs cause (phzsiological) of increase and decrease of cerebral and gray matter volume is ths water content in tissue dependend of hydaratation and dehidratation  of body and brain (osmolarity):  The brain is living tissue, bat the CT and MRI scan is only static pictures!

While lacking a proper reference, I'd hypothesize it's similar to activity in- and decreasese seen for learning tasks. An increase in activity can depict the utilization of more regions/connections whereas a decrease can indicate efficiency. Meaning that both an increase and a decrease in activity can indicate learning improvements. The same could be true for gray-matter plasticity.

I must however say that I have personally not yet encountered a paper reporting gray-matter reduction after learning. Do you perhaps have an example showing such findings?

Gray matter is mainly cell bodies, not axons or schwan cells. During your life some neurons die, due to age, nutrition, disease or lesions. So yes, your gray matter reduces. One less known fact is that there is neurogenesis (birth of new neurons) in the brain during life, especially in the hippocampus and the olfactory bulb, so yes, your gray matter increases during life.

Now, experience can work either way, by expanding or reducing the dendritic branch and the synaptic connection among neurons. In other words, when you learn a new thing you tend to form new connections, in a way you can think that as branches of the neuron growing to connect to new ones. And as you sleep (or consolidate) it is thought that there is a selection process of the stronger connections (in part because there is not infinite resources to keep all the synapses alive) enhancing them and retracting the weaker ones.  

Synapse competition is the field of study of this process and it is very fascinating.

That said, I have no data about new neurons being formed or destroyed solely based on experience.

@ G. Mello: Gray matter is NOT mainly cell bodies! Pericaria make up less than 3 % in volume in grisea, glial cells about 1 %, the rest is dendrites and axons.

To my knowledge absolute increases in grey matter are only seen in oedema, reversible reduction in volume only in dehydration. Cortical fields can show relative increase by extensive training ("remapping"). This is because existing neurons are recruited for a specific task, not because of new neurons.

One normal example of reduction in gray matter is pruning in the early developmental period, a process which appears to be reduced in autistic children. While abnormal reduction in gray matter is often seen in cerebral atrophy in a variety of dementias noted among the aged.

Might the increasing and decreasing have anything to do with the distinction between adaptation and habituation?

Gray matter consist of cell bodies and many synapse. In learning processes occur synaptic plasticity processes. In synaptic plasticity processes, two or three kinds of changes occur. First, in presynaptic region occur biochemistry changes where increase production of neurotransmitters. Neurotransmitters are packed in vesicles. Second, changes in anatomy where increases of terminal button and maybe also axon terminals to support terminal button. In post synaptic region to receive increases of terminal button, the number dendritic spines also increase and also membrane density of the soma that receive terminal button synapses.  Third, in biochemistry and anatomy changes many new protein are produce that need the role of DNA. So the gray matter can increase its volume. But the synaptic plasticity not always in positive direction. If the neuron circuits were not used again, neurotransmitters, terminal buttons axon terminals, dendritic spines and membrane density will decrease. This will make the gray matter decrease in volume to the normal condition. So the changing processes in the synapse is called plasticity because the synapse can change like a plastic. That is what that I learned from many books.

Increase and decrease of cerebral gray matter volume is the wellknown scientific and clinical fact, which have the name -neuroplasticity . Gray matter, s volume depends on  cortical activity and presence of pathology. We have finished the research: Voxmetry of cortical volume and sucortical tracts in patients with traumatic brachial plexus axothomy. It is revealed the Increase of cortical volume and some sucortical tracts in case of recovery and  the decrease of them in case of unfavorable course of the disease. So yes, there is neurogenesis in the brain during recovery, not only in the hippocampus and the olfactory bulb. The intensity of these processes depended of age and degree of injiury. I hope the article will be soon published. 

@ Sergey Anatolievich Zhivolupov 

if You can prove that there is neurogenesis outside bulbus olfactorius and hippocampus, You deserve the Nobel prize. Good luck.

I must repeat!

The first cause (physiological) of increase and decrease of cerebral volumen and gray matter volumen is the water content in tissue dependend of hydaratation and dehidratation  of body and brain (osmolarity):  The brain is living tissue, bat the CT and MRI scan is only static pictures!

Why does gray matter volume increase and decrease? - ResearchGate. Available from: https://www.researchgate.net/post/Why_does_gray_matter_volume_increase_and_decrease [accessed Jul 20, 2015].

@ Nikola Ilankovic: why must You repeat? Saying or writing things twice does not improve the quality of the information.

Cognitive and neuroscientific research over the past 60 years has shown that cognitive abilities, and their underlying neural architectures, change depending upon the experience of learning itself (Raisman, 1969; Cicchetti and Blender, 2007).

Here is another paper/citation I hope you find useful:

Driemeyer, J., Boyke, J., Gaser, C., Büchel, C., & May, A. (2008). Changes in Gray Matter Induced by Learning—Revisited. PLoS ONE, 3(7), e2669. http://doi.org/10.1371/journal.pone.0002669

Abstract

Background

Recently, activation-dependant structural brain plasticity in humans has been demonstrated in adults after three months of training a visio-motor skill. Learning three-ball cascade juggling was associated with a transient and highly selective increase in brain gray matter in the occipito-temporal cortex comprising the motion sensitive area hMT/V5 bilaterally. However, the exact time-scale of usage-dependant structural changes occur is still unknown. A better understanding of the temporal parameters may help to elucidate to what extent this type of cortical plasticity contributes to fast adapting cortical processes that may be relevant to learning.

Principal Findings

Using a 3 Tesla scanner and monitoring whole brain structure we repeated and extended our original study in 20 healthy adult volunteers, focussing on the temporal aspects of the structural changes and investigated whether these changes are performance or exercise dependant. The data confirmed our earlier observation using a mean effects analysis and in addition showed that learning to juggle can alter gray matter in the occipito-temporal cortex as early as after 7 days of training. Neither performance nor exercise alone could explain these changes.

Conclusion

We suggest that the qualitative change (i.e. learning of a new task) is more critical for the brain to change its structure than continued training of an already-learned task.

Furthermore researches with comparation anything with anything without functional or etiopathogenetic relationship, and then in structural view...

@ Nikola Ilankovic: sorry, but i don't understand your answer.