I want to know the logic behind such comparison. Same is being done by BtReannouncer.net
Do the torrents have similar hash or do they compare it by size and name?
Most torrents on piratebay do not have trackers other than piratebay, but torrentz.com provides a complete list of all the trackers that are tracking the same torrent.
Each torrent file contains a hash id which identifies it as unique. As this torrent is passed around to more trackers, the hash id stays. It's also frequently known as an info hash.
febd9a2cb755ec82e6e7a015a8dc497fde9dd507 would be an example hash id.
If you Google it you'll notice that it shows up at many different trackers. Spidering search results and checking in on the major torrent sites can let you index which sites host the torrents, then correlate the intersecting info hashes (Something like, SELECT trackers FROM torrents WHERE info_hash="xxx"). So you'll have torrents with the same info hashes from The Pirate Bay, Mininova, etc.