Idle power seems to bear this out: 6.8W for the smallest, 7.8W for the 146GB, and 10.1W for the largest. The difference in power is roughly proportional to the difference in number of platters.
So, why are all three drives listed with an identical weight of 0.726kg? (For example 73GB and 300GB.) Surely a disk platter must weigh at least 1 gram.
I think they rounded off the weight to 1.6 pounds and did the metric conversion afterward. Further evidence for this theory is the listed depth of "5.75in/146.05mm" on Seagate's spec sheet.