Uploaded files are processed on our own conversion servers, nobody looks at them, and they are not shared. The upload and the converted file are both deleted automatically about 24 hours after the conversion, which leaves time to download the result. See /privacy/ for the full policy.
It depends on the formats. A lossless target such as PNG, WAV or FLAC keeps what the source holds; a lossy target such as JPG, MP3 or MP4 is usually re-encoded, which trades some detail for a smaller file.
