Diapers!!

My 3 week old baby is in size newborn diapers still and every time I change him he has lines on his skin around his thighs and butt from the diaper as if the diaper is too tight but I literally don’t even strap it tight i do it loose bc I thought it was bc I was tightening the straps too tight. I put my finger in the front of his diaper to feel if it’s tight or a good fit and it doesn’t feel tight so I don’t think he’s ready to go into a size 1 yet but I’m so confused of why he’s getting these lines on his body if the diaper doesn’t feel tight. He weighs 10lbs if that helps..