Jason Kelce says Taylor Swift has had a "staggering" impact on Travis Kelce ... revealing his little brother has grown up tremendously since he began dating the pop star.

Jason got candid about the Chiefs tight end's relationship with Taylor on Tuesday's episode of the "Fitz & Whit" podcast ... explaining to former NFL stars Ryan Fitzpatrick and Andrew Whitworth he's noticed a huge change in his bro since he began shacking up with Swift nearly two years ago.

"It's been staggering, honestly," he said.

Jason noted his brother used to be like "Peter Pan and the Lost Boys," but now, "it's changing."

"I think sometimes you get people in your life that maybe get that out of you," he said, subtly referencing Travis' partnership with Swift. "And that's a good thing."

The whole convo was actually sparked by Fitzpatrick's own observation of Travis ... as he told the guys on the show that when he visited the three-time Super Bowl champion prior to his relationship with Swift, "I don't think he had any couches" in his home.

Jason, however, insisted Travis' whole persona hasn't entirely changed ... saying on the pod he still "has got that youthful enthusiasm."

Whether the 35-year-old gets to show it for one more year on the gridiron remains to be seen ... as he's said he's considering retirement following K.C.'s Feb. 9 Super Bowl loss to the Eagles.