There's an old joke about a person being asked, "How many people work in your office?" The person responds, "About half of them."

This punchline comes to mind when I look at the S&P 500. Many of the stocks in the index don't perform all that well over time. But as the more-successful stocks outperform, they earn an increased weighting in the S&P 500 because of their larger market caps.

Which stocks in the S&P 500 will work the most for investors throughout this decade? It's impossible to know for sure. However, I'll make a prediction: The following three S&P 500 stocks will at least double in seven years.

1. Amazon

The larger a company grows, the harder it can be to deliver the same rate of expansion. But that doesn't mean really big companies can't grow significantly. I think Amazon (AMZN -0.17%) has proved this point in the past and will continue to do so.

When asked about Amazon, the first thoughts of many individuals would probably be about the company's online shopping platform or its Prime Video streaming service. My view is that both could be solid growth drivers over the coming years. But they won't be the most important factors in enabling the stock to double.

Instead, that honor belongs to Amazon Web Services (AWS). As much as 95% of worldwide IT spending goes toward on-premises hosting rather than in the cloud. CEO Andy Jassy expects "the equation is going to shift and flip" over the next 10 to 15 years with a lot more spending on cloud hosting versus on-premises hosting. If he's right (and I think he is), Amazon is a no-brainer stock to buy right now.

AWS already ranks as the biggest cloud-hosting provider. It's also Amazon's most profitable segment. The company's profits should explode by the end of the decade with the transition to the cloud. My confidence level is pretty high that Amazon's share price will at least double within seven years or less.

2. Digital Realty Trust

Digital Realty Trust (DLR 0.37%) isn't the household name that Amazon is. However, the company should benefit from the same trend that Amazon will.

Digital Realty Trust owns more than 300 data centers. The transition to the cloud should be a key growth driver for the company.

A quick glance at Digital Realty Trust's top customers reveals a Who's Who in the technology world. A long list of major cloud providers, software specialists, social media companies, and telecommunications giants use Digital Realty Trust's data centers.

If you only look at Digital Realty's stock performance over the last 10 years, you might doubt that it could double by 2030. But it's important to consider total returns rather than share-price appreciation alone.

Digital Realty Trust is a real estate investment trust (REIT) and must return at least 90% of its income to shareholders to avoid paying federal taxes. Its dividend yield tops 4.8%. With that high yield, the stock won't have to deliver huge gains for Digital Realty Trust to generate total returns of 100% or more over the next seven years.

3. Vertex Pharmaceuticals

I think that Vertex Pharmaceuticals (VRTX 1.36%) is another S&P 500 stock with a clear path to doubling or more by 2030. The company already enjoys a monopoly in treating the underlying cause of cystic fibrosis (CF).

Vertex could increase its market by roughly 50% by securing additional approvals and reimbursement deals for its existing CF drugs and by achieving success with its experimental messenger RNA CF therapy VX-522. 

But Vertex has even greater growth opportunities beyond CF. It hopes to win regulatory approvals for exa-cel, a gene-editing therapy developed with CRISPR Therapeutics, as soon as later this year. Exa-cel could generate peak annual sales of at least $2 billion in treating sickle cell disease and transfusion-dependent beta-thalassemia.

Non-opioid pain drug VX-548 could also make it to market within the next couple of years. Vertex believes that this therapy has multibillion-dollar potential.

The big biotech is also making good progress in its clinical testing of inaxaplin in treating APOL1-mediated kidney disease (AMKD). There are more patients with AMKD than there are CF patients.

Vertex could have other major catalysts over the next few years as well, notably from progress with its clinical programs that could hold a cure for type 1 diabetes. 

Biotech stocks face the risk that their pipeline programs could flop in clinical studies or fail to win regulatory approvals. But my view is that Vertex has enough arrows in its quiver that it will be able to double investors' money within the next seven years.