The Russell 2000 Index is usually considered a surrogate for Small Cap Growth issues. And, according to Bloomberg, the two indexes have performed differently. This was a very interesting time for the indexes, since it includes a bull market run-up in overall stock valuations, followed by a significant decline. Indexes move both ways, of course. 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.