I find their rank system work in the way that the less file u have, the easier to get higher rank.
The ratio is determine by the number of file u upload as well as the number of time other download.
On top of that, it also will add number of sales u generate by the file.
So total of three factor.
I remember i have copper for a long time about 3TB of file. And my file got wiped. Mostly those not download for a long time. I move around the file and deleted most of the not popular file, and now i am in gold with 750GB of file..

Hope this help.