What's the real meaning of "brain shrinkage"?
Brain shrinkage = brain atrophy, which means a decrease in volume of the brain tissue, which involves different types of degeneration on the cellular level.

Here is a Wikipedia article about cerebral atrophy with a list of ~30 causes, such as Alzheimer's disease, stroke, cerebral palsy, malnutrition, AIDS, etc. Mechanisms can involve: loss of cytoplasmatic proteins, loss of complete neurons or connections between them, etc.

Alcohol consumption and frontal lobe shrinkage: study of 1432 non-alcoholic subjects (BMJ):


  The brain tends to shrink physiologically with age. Heavy alcohol
  consumption seems to exaggerate this shrinkage in social drinkers.
  Moderate alcohol consumption does not seem to affect brain volume.


Vitamin B12, cognition, and brain MRI measures (Neurology):


  Methylmalonate, a specific marker of B12 deficiency, may affect
  cognition by reducing total brain volume...