What they both said. If it has higher density platters, it will be faster until it fills to the point where the data is going to the other side of the platter. I think it's faster when the data is being written to the outside edge because that is moving past the heads faster than the smaller diameter inside edge.
If you have the patience, Anand's second SSD article at Anandtech has very useful and interesting info of that type. Warning:very long.