I did not expect to see the day that Warren Buffett's Berkshire Hathaway would underperform the S&P 500 in 5 out of 6 years, but that's basically what has happened. On the other hand, over the last 7 years (and also for any period longer than this), Berkshire has still handily outperformed this same benchmark! This is the more important indicator, in my opinion.
Berkshire's businesses are cash flow generators that can be expected to rack up growth in book value for the firm both in good times and in bad. But looking at the S&P 500 over just the last six years leaves out the fact that the S&P 500 had a lot of "potential energy" (to borrow a physics term) over this period thanks to its ridiculously bad performance in 2008. Leaving out that bad performance, which acts as a springboard for future returns, biases the comparison.
Berkshire's book value fell only 10% in 2008, so over the full cycle it is still outperforming the market despite its size acting as a retardant.