That, of course, is the whole point of having exposure to both. Stock prices tend to be driven by earnings growth. The smaller the base, the easier it can be to post an impressive gain. Often smaller companies have a single, limited product. The market for their particular widgets may behave differently than the broad economy.
Smaller companies tend to be more capital-intensive.
Companies with smaller market caps can be less liquid, making them harder to buy or sell. There's a flip side, of course. Many of the same factors that lead to Small cap outperformance over time also make these issues more volatile. In plain fact, a number of elements can drive stock prices. Big price changes can result when lots of folks start to like a group of stocks or loathe them.