A group of researchers at UCLA decided to take high-res brain scans of people who have meditated regularly for more than five years, and they found that certain parts of meditators' brains are larger than average. Their brains have a bigger hippocampus and more gray matter in the orbito-frontal cortex, thalamus, and inferior temporal lobe—all areas that are involved in processing and regulating emotions.
This doesn't surprise, given that research has long shown that people who regularly practice meditation can focus better and have greater control over their emotional response to stress. Finding the bigger brain regions might give scientists a clue as to why meditators have these special abilities, says Eileen Luders, a postdoctoral research fellow at the UCLA Laboratory of Neuro Imaging who led the study. Little is known about the link between meditation and brain structure.
For one thing, scientists still face a chicken-and-egg question:

Because this was not a longitudinal study—which would have tracked meditators from the time they began meditating onward—it's possible that the meditators already had more regional gray matter and volume in specific areas; that may have attracted them to meditation in the first place, Luders said.

However, she also noted that numerous previous studies have pointed to the brain's remarkable plasticity and how environmental enrichment has been shown to change brain structure.