Disk capacity may have increased, but disk platters are exactly the same size as before: 3.5 inches. So, moving the read head around should cost mostly the same as before. The only factor I can think of is that the head has to be more precisely positioned, and that may (or may not) be more costly because of physical limitations (rebounds).
On the other hand there are two factors that should make seek time decrease: improved machinery and more density. More density means that more data goes faster under the read head, so more often seeks can be satisfied without moving the read head, just waiting for the data to pass below.