I'm still a little unsure if the numbers do signify how much power they have in OXIII by a few particular things:

1. In CoM, Vexen said that he, number IV, refused to take orders from Marluxia, who was number XI. Yet Marluxia was able to push him around. Though it may be because he was put in charge of Castle Oblivion, this brings up another question: Why would Xemnas give control of it to such a low ranked member; maybe rank might not matter so much?

2. In KHII, it seemed that Saix (who was ranked quite low at VII) had much more power than the other members because he always appeared in scenes with Xemnas (much more than any other member) which made me think that he possibly could have been Xemnas' right hand man. Not to mention that almost every other member had been sent to eliminate Sora before he had (Saix was the second to last OXIII member to die, before Xemnas). I would typically think that the weaker villains are sent to eliminate the main hero first, then the stronger ones after.
And Marluxia has that big thing at the end of CoM...£10 bet it's called a reaper for something like that.
I think I saw an interesting theory to that on KHinsider. It had a theory that Marluxia himself WAS the big "reaper" nobody, since every time his human form was defeated, it turned out to only be an "image."