Portland had the second-biggest rise in rents among the country’s 100 biggest metropolitan areas, according to the latest data from Zillow, an online national real estate firm.

The company released data Tuesday showing the year-over-year percentage increases in median rents for 862 metro areas in May.

While rents rose 4.3 percent nationally, the median rent for the Portland area rose 17.4 percent, to $1,582 – $8 more than in Philadelphia and $38 less than in Chicago, according to Zillow’s data.

The increase from last year in Portland was second among all metro areas, behind only Jackson, Mississippi, where rents went up 22.7 percent in a year.

Portland is the smallest of the top 100 metro areas tracked by Zillow and the only metro area in Maine among the top 100.

Smaller Maine metro areas also had rapid rises in rents, from a 14 percent increase in Augusta to a 24.3 percent jump in Rockland.

Skylar Olsen, a senior economist for Zillow, said the national trend toward rising rents is a result of the improved job market enabling people who lived with their parents or roommates during the recession to “spin out” and form their own households.

In Portland, where the unemployment rate is below the national average, “it’s just a little bit more extreme,” she said.

On top of that, she said, “it’s just a very nice place to live.”

The rise in Portland rents actually slowed down in May. In March, the year-over-year increase was 21.6 percent, Olsen said.

She said that means either more units have come onto the market or the demand for them is “pulling back,” because the prices have gotten too high.

“You can only accept so much more rent before you can’t afford any more,” she said.

Jeff Levine, Portland’s director of planning and urban development, said Zillow’s data is similar to figures the city has been seeing, showing about a 15 percent increase in rents over the past two years.

“The City Council and staff have been working on policy solutions to assure that, as the city grows, the housing developed serves a wide range of residents,” he said.

Levine said the focus is on so-called workforce housing that would enable a family of four earning $77,000 a year to keep their rent around 1,800 a month, 30 percent of their income – what’s generally accepted as affordable.

“We are seeking to achieve this goal by both encouraging new housing investment to increase the supply and ensuring that the housing developed meets the needs of Portland residents,” he said.