NEW YORK ( TheStreet) -- Wal-Mart ( WMT - Get Report) has a big problem. It's getting too big to invest in.

Over the last year, the stock is up less than 8%. Shares in rival Costco ( COST - Get Report) are up 21%, Target ( TGT - Get Report) is up 24%, and Kroger ( KR - Get Report) is up nearly 64%.

Those three retail rivals, between them, are just 60% of Wal-Mart's size. With nearly $470 billion in sales last year, the needle is just hard to move for Wal-Mart.

Consider. Between 2011 and 2012 Wal-Mart sales grew about $23 billion. Sounds great, but it's barely 5%, the same growth rate as Target. Kroger sales grew just $6 billion, but that's 6% growth. Costco's $10 billion sales gain was still 11% growth.

Then there is the cost of maintaining that size, and engineering that growth:

All this publicity might make you want to take the whole company private.

The thought may be occurring to founder Sam Walton's kids, who have kept stakes in the company through Walton Enterprises LLC. The trust now owns almost 49% of the common stock, family members themselves own another 1.29%, and when the latest $15 billion stock buyback is done those percentages will rise.

This makes Wal-Mart a "controlled company," meaning it's no longer required to have an independent board. The company denied it would start running that way at its annual meeting last month, but the board is shrinking, the percentage of outsiders going down.

At some point, if the law of large numbers discourages investors, if the stock buybacks continue, we could reach a point where the family decides a tender offer for the entire company makes sense.

There are large private companies that grow much faster than public ones. Privately-held Koch Industries managed to double its revenue from 2000 to 2011, expanding company-wide employment fivefold, as the Wichita Eagle reported with pride.

If the Waltons can grow their company faster as a private entity than a public one, why wouldn't they?

At the time of publication, the author was long COST.

This article is commentary by an independent contributor, separate from TheStreet's regular news coverage.

Dana Blankenhorn has been a business journalist since 1978, and a tech reporter since 1982. His specialty has been getting to the future ahead of the crowd, then leaving before success arrived. That meant covering the Internet in 1985, e-commerce in 1994, the Internet of Things in 2005, open source in 2005 and, since 2010, renewable energy. He has written for every medium from newspapers and magazines to Web sites, from books to blogs. He still seeks tomorrow from his Craftsman home in Atlanta.