There are two primary reasons that I see people use smalle rdrives.
1. They already have the drives. That'd be true in my case. Already had two 40gig Barracudas, don't needd any more space than that, love the quietness and reliability, so I didn't waste money on new drives.
2. They are using the WD Raptors, either 36 or 74gigs, and they have those small capacities cuz that's as big as they go.