If you say
He is a big boy.
You are literally referring to something about your friend, his size, his maturity, etc.
If you say
He's one of the big boys.
You are figuratively saying he is a big player and important here. For example, in investment banking the "big boys" are Goldman Sachs, Morgan Stanley, Merrill Lynch, Credit Suisse.
To describe his previous position, you might say
His served as a sales manager in a famous local bank.
His previous position was as sales manager in a famous local bank.
as evidence that he is "one of the big boys". However, usually the term "one of the big boys" is reserved for Managing Director or above.
One notes the increasing use of "big boys" during the bull market of the 80's
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