why be mobile "performance" HDD smaller capacity?
 

I had a laptop with 2 WD Scorpio Black drives, and it seemed quite
impressive. That was only 750 GB. Seagate's rival Barracuda Pro goes
up to 1000 GB. Whereas a plain 2.5 inch drive goes up to a 2 TB
from WD, and a fat 5 TB from Seagate.
What is compromised to get the speed?