Apples meteoric growth has been well documented. But the companys outsized weighting isnt making the S&P 500 as top-heavy as one might think.

Several strategists have been stripping Apple out of the equation and recalibrating their earnings expectations for the S&P 500 minus the tech heavyweight. The idea is to create a better sense of the broad market without one company skewing the results.

But Brown Brothers Harriman rejects the notion that the S&P 500 is looking top heavy these days. Instead the firm slices the index into groups of 50 stocks by overall size and concludes the biggest stocks arent throwing the overall index out of whack.

The combined percentage of market value of the top 50 stocks in the S&P 500 is almost exactly at its historical average of 51%, Brown Brothers says. By comparison, this figure surpassed 60% during the tech boom in the late 1990s.

The market is arguably not nearly as top heavy as it was in the 1990s, but Apple has grown to a proportion of the benchmark not often seen for a single stock, says Sam Burns of Brown Brothers Harriman.

To be sure, the Apple effect isnt something that should be ignored. Apple, which earlier this year took the throne as the worlds most valuable company, has a weighting of more than 4% in the S&P 500.

When comparing Apple to other top S&P 500 companies throughout the years, the current top dog is in the upper end of the historical range since 1990, Mr. Burns says.

Apple shares recently rose 0.7% to $632.88. The stock has jumped more than 55% in 2012 and surpassed $600 earlier this month.

The companys market cap touched $600 billion yesterday for the first time ever. It only took Apple 28 trading days to add another $100 billion in market cap from the first time it touched $500 billion, according to WSJ Market Data Group.

Earlier this year Apple and Exxon Mobil were jawboning for the title of worlds most valuable company. But Apples market cap is now about $200 billion larger than Exxons, or roughly the size of IBM.

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