If your company put stock options or restricted stocks in your 401K acct and the stock price goes up 50X, you end it up with 3 mil! Or if you have started putting money in 30 years ago, you have good chance of having more than 3 mil.

By the way, the average size of 401K is around 90,000. Be happy if you have more than that! If not, try to save more!