The spike in home runs has happened relatively quickly. Baseball last expanded in 1998, and starting that year there were at least 5,000 home runs every year for nine seasons. But in the next nine seasons, through 2015, only one featured at least 5,000 homers.

The low point in the last two decades was 2014, when just 4,186 home runs were hit. The total climbed to 4,909 in 2015, then 5,610 last year, making for a curious trend line. An undetected, large-scale increase in performance-enhancing drug use could still be an explanation for the current surge in home runs, but the stricter testing and penalties in place would argue against that notion.

“If you want to use the 2000 analogy, we know what was going on there,” said the broadcaster Al Leiter, an All-Star left-hander for the Mets in 2000 who neutralized power hitters with inside cutters, keeping opponents from extending their arms at the plate.

“The assumption now is that steroid testing is legit, and it’s working, so a lot of it has to do with approach,” Leiter added. “You don’t see two-strike approaches anymore. Guys don’t care if they strike out, and it’s basically turn and burn. It used to be turn and burn for three or four guys in the lineup. Now it’s seven.”

Leiter, who calls games for the YES Network and works as an analyst for the MLB Network, said too many players who should be hitting eight to 10 homers per season were now hitting 15 to 20. Bert Blyleven, the Hall of Fame pitcher and Twins broadcaster, echoed Leiter, saying that even bottom-of-the-order hitters are willing to risk strikeouts for the chance to go deep.

But, Blyleven added, the problem really starts on the mound.

“I would have to say the bottom line is the lack of quality pitching,” he said. “We see a lot of it. A lot of guys come up so quick, all they do is throw as hard as they can. I don’t care how hard they throw, if the ball’s right there, these guys are going to hit it.”

As for the balls themselves, Major League Baseball has said repeatedly that extensive testing has shown them to conform to normal specifications. But if the balls are wound tighter — even within an acceptable range — they could be harder, with lower seams, and less prone to resistance when airborne.