Insurance companies are piling into exchange-traded funds.

Over a third of U.S. insurance companies were invested in ETFs in 2018, according to a survey conducted by analytics firm Greenwich Associates. In total, they held 372 million shares, up 7% from the prior year.

The uptick was driven in part by mega-cap companies — those with over $50 billion in admitted assets, as defined by the analysts conducting the survey — which increased their ETF-invested assets under management by 39%, even as total assets under management in this area declined by 3% due to market volatility, the survey said.

"It's really exciting to see the mega-cap insurance companies move more aggressively into using ETFs," Todd Rosenbluth, senior director of ETF and mutual fund research at CFRA, said Wednesday on CNBC's "ETF Edge."

They largely gravitated towards ETFs with high liquidity profiles, or the ones whose assets are most easily converted to cash, Rosenbluth said.

Their most popular investments included:

What this told Rosenbluth was that these companies are "using the broader tools that are out there in a market-cap-weighted way and getting exposure," he said. "That's really going to be a confidence-booster for investors, when they want to be able to get out, to know that there's a large institutional investor that's on the other side of that trade to execute that."

Matthew Bartolini, managing director and head of SPDR Americas Research at State Street Global Advisors, agreed that this development will benefit the average investor.

"By having them trade the ETFs, that's going to increase the liquidity profile for all different investors and lower overall trading costs, which are really an important part of that total cost of ownership," he said in the same "ETF Edge" interview.

And, as of now, there's still room for this side of the ETF market to grow, Bartolini and Rosenbluth said. Rosenbluth pointed out that insurers currently have less than 1% of their overall asset base invested in ETFs, meaning there could be meaningful increases ahead.

"That needle can really move if we have some of these larger insurance companies moving in," Rosenbluth said. "That will really drive overall liquidity and make sure that there is an investor on the other side of the trade when a retail investor wants to exit their existing position instead of having to worry if there'll be problems."

And, by Bartolini's estimates, "the rate of growth is going to continue" in a big way.

"We've seen a pickup from 2016 to 2018, so ... the percentage is probably going to increase in that linear fashion," he said, adding that he could see it growing to 2% to 5% in the next two or three years.