Quick question - if a picture contains size_difference, but only contains two characters of the same gender, isn't tagging smaller/larger redundant? I'm more than happy to go through the tags and remove them if so (going through male/male smaller_male, male/male larger_male etc).
Maybe I'm wrong and they should be tagged anyways, but surely the reason you would search with those tags would be to find a character of one gender smaller than another? If the picture is female/female, there is both a larger female AND a smaller female so the tags are useless. The same search could be completed with "size_difference female/female".
The thing is, it would probably take a while and it doesn't really affect much - whilst I'd be happy to put in the leg work, if you don't want same sex pairings when searching for size difference you can just use "-female/female". Is it worth going through and removing?
Updated