Alphabet Inc.'s (GOOGL) Google and Facebook Inc.'s (FB) dominance of online advertising has swelled to the point that the two companies now risk “saturating available digital budgets,” according to one Wall Street analyst.

In a research note, reported on by Barron’s, Pivotal Research Group estimated that the tech giants now account for 73 percent of all U.S. digital advertising. The brokerage came to this conclusion after analyzing new data from the Interactive Advertising Bureau showing that digital advertising grew 23 percent in the second quarter of 2017 to $20.8 billion in revenue. According to analyst Brian Wieser, it’s likely that Facebook and Google accounted for a whopping 83 percent of that growth. (See also: Google Ads Vs. Facebook Ads.)

Rather than view that progress as a reason to be even more bullish on the stocks, Wieser urged investors to be cautious. In the research note, he warned that the duopoly now risks oversaturating a market characterized by limited budgets.

"Growth rates are remarkable, but also highlight risks around digital media owners — and Facebook and Google in particular — saturating digital media budgets in years ahead, and reinforce our cautious views on stocks in the sector at current levels," Wieser wrote in the note. (See also: Amazon Could Erode Facebook, Google Ad Dominance.)

Key to the analyst’s skepticism is his observation that advertising budgets are rarely tied to return on investment-based metrics, even if investors continue to think otherwise. Instead, Wieser expects advertising spend to be “constrained,” moving in line with the direction of the economy and general consumption patterns.

“Although the presence of digital media owners makes it more possible for some types of advertisers to emerge (app developers in particular) and support some incremental growth, the bulk of the advertising “pie” is constrained, because in our view, absolute advertising budgets are rarely tied to pure ROI-based metrics as many investors might expect them to be,” he wrote. “Budgets are set with some correlation to a company’s revenue, and in turn those budgets are somewhat closely [tied] to the health of the broader economy, and personal consumption expenditures in particular.”