Are you looking to grow your stock portfolio beyond the million-dollar mark? That's certainly a big, yet achievable goal. The chief challenge to getting there is simply maintaining the right collection of tickers along the way. You need some growth stocks to be sure, but there's much to be said for the predictability of value stocks. While you typically know exactly what you're getting with large-cap stocks, smaller companies offer more explosive potential. Picking the right tickers can be tricky.

To this end, warehouse club-based retailer Costco Wholesale (COST 0.48%) is in the large-cap sliver of value stocks; it offers slower growth in exchange for certainty. That's fine but not typically considered millionaire-making stuff (unless you're already starting with a tidy six-figure sum).

The fact is, however, this name's got more growth firepower than you might expect for one underappreciated reason.

A long history of progress and success

It's rather impressive when you think about it. More than 64 million households (or more than 116 million individuals) pay Costco for the privilege of spending money at one of its 838 stores despite the likelihood that a Kroger (KR 0.77%), Walmart (WMT 0.09%), or Target (TGT 0.76%) store is just as accessible. Costco's prices are typically lower than its competitors. Since consumers can step foot in one of those other retailers' stores for free, its prices should be lower.

Even more impressive is the company's consistent growth. Since the late 1990s, the company's annual top line has swollen from less than $60 billion to more than $200 billion, while per-share profits have improved from less than $2 to more than $12. This year's projected revenue growth of nearly 16% and expected earnings growth of 18% are slightly stronger than their historical norms -- but only slightly. Clearly, the company is doing something right.

COST Revenue (TTM) Chart

COST Revenue (TTM) data by YCharts.

Veteran investors know nothing lasts forever, and nothing invites competition like consistent success. Names like Netflix and Intel are prime examples of this reality. Sooner or later (and often sooner than later), consumers and corporations should seemingly have more than enough choice in terms of getting plenty of bang for their shopping buck.

That point in time may be much further down the road than it feels like it should be.

The key to Costco's continued growth isn't rooted in what it is, but in what it isn't (at least not yet). What it isn't, yet, is everywhere its competitors are.

Smaller companies just grow faster

As noted above, Costco currently operates 838 membership-required stores, 578 of which are located in the United States. That's only a fraction of the 3,573 domestic supercenters Walmart currently boasts in addition to its 600 similar club-based Sam's Club stores. There are nearly 2,000 Target stores in the United States alone and nearly 2,800 Kroger locales when including all of its subsidiaries like Fred Meyer, Fry's, Harris Teeter, and King Soopers, just to name a few. All of these companies operate modest businesses outside of the U.S. as well, proportionally.

That seems like a massive amount of competition, perhaps even pushing the grocery and basic consumer goods businesses to the point of saturation.

The fact is, however, Costco's new-store growth has followed Walmart and Target's footprint expansion for years now, and the company's done just fine in markets where its chief rivals were already established. Indeed, last month's same-store sales growth of 7.6% within the United States is in line with its long-term norms, if not slightly lower. It's also far better than the typical same-store sales figures reported by outfits like Kroger and Walmart.

So it merits repeating: Costco is clearly doing something right.

The best of the best, plus...

Is Costco looking to erect a new store to compete with every single Walmart, Target, and Kroger locale? Probably not. It's built several hundred successful shopping warehouses in the same geographic markets at the same time its top competitors built several thousand stores, however. Assuming it continues using its proven, winning formula, look for the same sort of strong same-store sales growth and double-digit revenue growth it has produced for most of its history.

In other words, Costco's current (relatively) small size is its biggest and best growth edge.

COST Chart

COST data by YCharts

And this growth edge is a big deal to current and would-be investors looking to invest their way to a million-dollar portfolio. General merchandise and food retailing represent a reliable but typically not a high-growth business. Costco's membership-based practice makes it a high-growth one, though, that could keep on doing well into the foreseeable future.

Yes, owning Costco stock for the long haul can help you become a millionaire. The company's business model has allowed shares to dramatically outpace its competitors' stock-price progress, and there's no reason to think the company can't sustain this trend.