So I would say this isn't an easy thing to define to begin with.  For want of a better way of saying it, if you are a monster downstairs, everyone is tight, if you're teeny then everything is loose.

Compounding this is the fact that the "tightness" of a woman's vagina changes.  With a lot of activity especially from larger things it can loosed a bit, then tighten right back up within a reasonable time frame.  You may have noticed on occasion that a woman that started very tight at the beginning of a session may feel looser at the end.  If you've had a long term SO you probably noticed the same thing.  It's muscle, and it swells and changes like any other part of a woman's body depending on everything from the day of the month, to their salt intake, to how much water they drank, etc.  

How much the woman exercises her kegel muscles can also make a huge difference.  Some women can actually undulate the interior (it's an odd but extremely pleasant feeling)

How you experience the woman also makes a huge difference, even doggie style with legs open vs legs closed, vs legs crossed is a very different feel.  

For all these reasons and more I'd say the reason it isn't categorized is because it would be almost impossible to establish any kind of reasonable standard.  If this is something important to you, I'd instead suggest asking in a nice way on your local board.