Or that the smaller one is older and you're paying a price for that niche effect. (e.g. at the very low end, you can't get a 20Gb disk for $0.99, what you typically get (not right now though) is a large range of sizes, say 160Gb to 1TB, all within a few dollars).

1TB drives were first announced in Jan 2007, nearly 5 years ago, is that long enough to push them out of the main manufacturing effort?

and faster seek.