As with many high performers he did it by fantastic returns at some point. I just looked up his returns in a business insider article and a quick calculation indicates that although someone who invested year one got rich, his returns over the past 30 years average to 6% (by my calculation- willing to be corrected)

I recall reading a stock book back in the 1980s that claimed that Buffet's legendary status at the time was entirely due to his purchase of Geico