It's a tiny bit bigger. Also the regular 15 has the UW camera of iPhone 11(!!!), meanwhile the 13 Pro has a much brighter UW lens, it also features the same 3x camera as the regular 15P.
So overall I would say the 13P still features a better camera set-up, altough you will get more detailed/better shots from solely the main camera of the 15.
Feature-wise I would say it's a tie.
You will get the new portrait mode, action mode, 4k cinematic mode on the 15.
The 13P on the other hand offers ProRaw, Macro, nightmode portraits, better zoom, 3x portraits/cinematic mode, lidar.