The stock market rally this week catapulted many stocks back to new highs, but there are still some relative bargains out there in big-cap technology worth buying on sale. For instance, picking up shares in (AMZN -2.22%), Twitter (TWTR), and Apple (AAPL -2.05%) could be smart if you want to own three industry titans that could see their revenue and profit head higher.

Its not as big as you think

Amazon seemingly has its hands in every market, and that could have you thinking it's pulled all the levers it can to deliver growth to its investors.

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While it's true that Amazon's become the go-to online retailer, e-commerce itself still only accounts for less than 10% of all retail spending in the U.S. Furthermore, sales at top brick-and-mortar competitor Walmart are double that of Amazon, suggesting there's plenty of retail market share still to capture.

Also, Amazon's got a great opportunity to grow revenue by winning away market share in electronic devices, such as virtual assistants and tablets. Amazon's tablet market share is less than 7%, according to Statcounter, and its virtual-assistant market share is below 50%, according to Strategy Analytics.

Similarly, Amazon Web Services (AWS) may be the big Kahuna in the infrastructure-as-a-service market, and Amazon Prime is a leader in entertainment, but Amazon doesn't have those markets all to itself. It competes heavily with top companies like Microsoft in cloud-technology services and Netflix in content streaming.

In short, Amazon can still deliver robust sales and profit growth from its existing businesses without relying on new markets. That's impressive when you consider that its revenue and net income grew 17% and 125%, respectively, year over year, to $60 billion and $3.6 billion, respectively, in the first quarter.

A top content curator

Twitter's business has had more ups than downs lately, especially compared to Facebook. Yet Twitter's shares are trading more than 15% below where they were one year ago, and that suggests investors may be undervaluing its potential to capture market share as we head into the all-important U.S. election next year.

Twitter allows you to engage with a broader audience by connecting you to the global news and content that's important to you. For example, connecting with industry thought leaders has made it one of my favorite ways to discover how healthcare experts view the latest clinical drug trials and drug approvals.

It's a must-see source for political insight, too. President Trump uses Twitter often to communicate his latest thinking, and increasingly, more politicians are embracing it so they quickly connect with constituents.

A crackdown on offensive content and new features, such as live video and audio tweeting, could boost user engagement, driving revenue up from its current pace of $787 million last quarter, making it another top tech stock worth picking up now.

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Much more than smartphones

Apple's iPhone is undeniably a top seller, but iPhone sales only represent 12% of all smartphone sales, according to IDC, making it only the third-largest player in the smartphone industry.  

Since smaller competitors control about 46% of the smartphone market, Apple can grow revenue by winning new iPhone users and convincing existing users to upgrade. Recently, its struggled to get existing customers to swap out their older phones, but faster speeds and new features may change that once 5G networks become widely available.

Apple could also see sales and profit improve because of other products. Mac computers only account for about 7% of total computer sales, according to Gartner. Plus, Apple's audio products, including Beats, and its Homepod virtual assistant, offer possible upside. Only about 25% of headphones used today are made by Apple, according to MIDiA, and Homepod's market share is only 4%, according to Strategy Analytics.

Importantly, the bigger Apple's user base becomes, the higher demand is for its high-margin services, such as apps and music. Last quarter, services accounted for $11.5 billion of its total revenue of $58 billion -- a record proportion. 

Finally, Apple's got an envy-inspiring balance sheet, giving it enough financial firepower to develop or acquire new products and pay a healthy 1.7% dividend yield. Given that Apple's shares are trading nearly 20% below their 2018 highs, now seems like a great time to add this titan to portfolios.