Currently, it seems all available torrents upload equally.  This gives a bias towards smaller torrents: if I have a small and a very large torrent, soon the small torrent will have a huge upload ratio, whereas the large one will still have a small upload ratio.

In order to prevent the extinction of large torrents, it would be good if uploading were done proportional to the size of the torrent, so that over time all upload ratios would be the same, barring other influences.

Another option would be to give preference to torrents with the smallest ratio, so that those other influences would automatically be accounted for as well.

Notes:
- In a way this is equivalent to the 'rarest piece first' rule in downloading.
- Of course other considerations might play a role, such as uploading more for torrents with fewer seeds, or more peers.  But lots of requests for a tiny upload might still be trumped by a single request for a huge upload: the relevant figure is the number of bytes requested by peers (and offered by seeds).