So we can extrapolate from your numbers that somewhere around 80% of the uploading is caused by 20% of the users. You know it's funny because you can actually find that same ratio in lots of places. I'd be willing to bet than in most if not all affiliate programs, 20% of the affiliates generate 80% of the sales.
We should totally document that pattern and name some sort of principle after it. Oh wait somebody already did
http://en.wikipedia.org/wiki/Pareto_principle