There is a lot to like about Realty Income (NYSE:O) and its hefty dividend. But one troubling trend must be watched in the coming years.

Realty Income has been on quite the run over the last 10 years, as its assets have grown from $1.7 billion to $10.5 billion. In this year's first quarter alone, it added more than $550 million in real estate.

While peer National Retail Properties (NYSE:NNN) has also grown, the following chart shows just how dramatically different this asset expansion has been:

Source: S&P CapIQ

It should come as no surprise that Realty Income has done a remarkable job in adding to its bottom line thanks to the impressive growth of its real estate assets. Its funds from operations have risen from $120 million in 2004 to $480 million this year.

National Retail Properties' bottom-line growth from $75 million to $230 million in the last decade is impressive, but that 200% rise is well short of the 300% mark registered by Realty Income.

So is this one more reason to consider buying Realty Income? Well, a dive into one more very important metric does raise a bit of a red flag.

The surprising direction
Instead of just looking at the bottom-line numbers, it's also important to track profitability -- and that's where a difference emerges. Although Realty Income has outpaced National Retail Properties in almost every oft-cited metric, a look at the return on assets -- measured by funds from operations divided by assets -- reveals a much different story:

Source: S&P CapIQ and Author Calculations.

As you can see, Realty Income held a massive lead in 2004, but as it has rapidly expanded its real estate portfolio through acquisitions, its ability to generate returns has only continued to fall. And while National Retail Properties has also fallen from 2004, it has been on quite a return-on-assets run since bottoming out in 2009.

What should investors make of this?

The key takeaway
The trend of falling returns is troubling. However, it is not necessarily a bad thing. After all, in 2009, 98% of Realty Income's rental revenue was from the retail industry, but by 2013, that had fallen to 80%. While leasing to commercial clients may be less profitable than retail, it is also likely less risky. In addition, with the massive acquisition streak Realty Income has been on, it could still be working through the kinks to ensure all the available profits are delivered to its bottom line.

Realty Income has many compelling things going for it, and one falling number isn't enough to reconsider investing in the company. But this is undoubtedly a trend that should be monitored into the future, as continued dips may present real challenges.