There is a limit at PW because you are uploading an mp3 file, and that tales space on the server, so we decided long ago on a limit. Now that the site has been bought out and disk space is probably no longer a problem - well, who cares at this point - I can't change it now anyway.
At PT you are not uploading anything to PT, but storing it somewhere else, so it doesn't matter.
But my experience at PW is that, if your submission takes more than a few minutes to listen to, you won't get as many listens... Or people will start listening and then move on unless it is very compelling or they have a special interest in the piece or the performer.