You can be prosecuted for piracy and many people have already been prosecuted for this. The more content you upload and download the more likely it will be that you are caught and charged. You draw more attention to yourself when you upload and download lots of data and because those who are searching for pirates want to prevent it they will go after the biggest offenders first.
As piracy becomes less common, even people who upload and download smaller amounts of content may be caught and charged. It is risky no matter how much content you are uploading and downloading.