Montgomery County leads the pack with a jump of nearly 10 percent, from an average of 1,768 inmates daily in 2009 to 1,940 in 2013, figures show.
Bucks County's average increased by about 9 percent over that time.
And while Chester County's statistics are not as linear, its average bounced between a low of 915 daily inmates in 2010 to a high of 990 in 2012.
Delaware County did not provide figures.
Experts offered a variety of reasons for why jail populations could be rising.
Joanne Metzger, an assistant professor of criminal justice at Temple University, said vigorous prosecution of drug-related crime in the suburbs may be related. And Alan Harland, an emeritus criminal justice professor at Temple, said "a huge part" of the problem is the amount of time people spend jailed while awaiting hearings for probation or parole violations.
"You can save huge numbers of jail-bed days . . . just by changing scheduling procedures and expediting processes," he said.
Montgomery County spent $23 million to add a wing for about 500 nonviolent offenders to its facility in 2010.
And nine years ago, Chester County spent more than $48 million to nearly double its jail's capacity, said county spokeswoman Rebecca Brain.
Construction is the approach Bucks is considering.
According to Plantier, the county's Department of Corrections developed a two-stage proposal, which will likely be discussed by the county commissioners at their meeting next week.
One step calls for spending $4 million to renovate vacant space and add 60 beds to a minimum-security women's facility the county operates on its jail campus near Doylestown.
A second step calls for spending $14 million to add an adjacent building with 100 more beds for female inmates.
Completing either phase would allow the county to bring back some female inmates who have to be jailed elsewhere when there is no room in county facilities.
It's not clear which option the commissioners favor.
District Attorney David Heckler - a former president judge - said he believes crowded jails make judges less likely to sentence defendants to prison.
"I think that becomes a part of the reality," he said. "It certainly had been articulated regularly by the members of the bench when I was attending the prison board meetings."
Plantier is simply hoping for more beds to alleviate overcrowding.
Both options, he said, would "give us some relief in a relatively short term."
BY THE NUMBERS
Daily inmates in Montgomery County in 2013.
Daily inmates in Chester County.
Daily inmates in Bucks County.
Delaware County did not supply a daily inmate number.