You become more diversified still by owning stocks from companies of different sizes: large cap, small & mid caps, international, etc.. (thecollegeinvestor.com)
It works best when you buy into multiple industries and include companies of all different sizes because this variety helps even out the ups and downs. (investor.vanguard.com)
You might also want to consider a stock's market capitalization before you invest, since companies of different sizes tend to behave, as a group, in different ways. (capitaloneinvesting.com)