The reason probably is because he is a lying murderer and used his pants for something nefarious in a murder knowing full well that when Pheobe is found, his pants will also be foundI'm not sure why you are all so hung up on the "brand new" trackpants that JW supposedly lent PB.
PB is 6 foot tall. That is highly unusual. She is slim, but not petite. I am also a 6 foot tall, slim female. My husband has a similar build to JW. I borrow his track pants frequently. His belly may be big but his hips and legs sure as heck ain't.
Also, millennials liked to wear baggy tops and tight bottoms. Gen Z are the opposite, preferring baggy bottoms and tight tops. A staple outfit for teen girls is a crop top or similar with big, baggy trackpants.
JMO