From Science Daily:
ScienceDaily (Mar. 3, 2009) — When it comes to the hippocampus, a brain structure vital to certain types of memory, size matters. Numerous studies have shown that bigger is usually better. Now researchers have found that elderly adults who are more physically fit tend to have bigger hippocampi and better spatial memory than those who are less fit.
The study, in the journal Hippocampus, shows that hippocampus size in physically fit adults accounts for about 40 percent of their advantage in spatial memory.
Read more ….