Are you uploading to AmazonS3 RootFolder? If so, that may explain the problem because for AmazonS3 the file is first uploaded to the server (memory) then to AmazonS3. So your server may be running out of memory (that may explain 2GB limit you are hitting) before actually starting to upload to AmazonS3. We plan to improve AmazonS3 upload so that it's directly uploaded from the browser to AmazonS3 in future versions.
For Physical RootFolders, there should be no problems even with a 15 GB file because it's directly persisted to disk.