Mid-cap companies sit in the middle of the corporate life cycle. They’re big enough to be proven, but small enough to keep expanding. That gives them more room to run than mega-caps, but also more bumps along the way. Historically, results vary depending on the index: MSCI’s last decade favors large caps, yet the S&P MidCap 400 has beaten the S&P 500 by about 2% a year since 1994.