You guys can argue all you want about subjective "size" and other bullshit, but let's put some numbers in here, shall we?

CORSAIR Voyager GT 128GB USB 2.0 Flash Drive
Dimensions: 4" x 1.5" x 0.67"
Weight: 0.22 lbs
Price/GB: $3.125

Western Digital My Passport Elite 250GB USB 2.0
Dimensions: 5.0" x 3.1" x 0.6"
Weight: 0.23 lbs
Price/GB: $0.24

Both draw power from USB. Both have similar performance in terms of transfer speeds.

If you ask me the answer is quite obvious. You are paying 13x more $/GB for the flash drive, whereas they have the same weight, same power requirements, and same transfer speeds. And the flash drive only takes 1" off the length and 1.5" off the width. That makes the flash drive half as small in volume, but that's still not very much, considering these things aren't that big in the first place.

That being said, I'm totally for SSD's and flash drives up to 32 GB. The higher capacity flash drives just aren't practical right now. In a year though I'm sure these numbers will all be vastly different.

P.S. I own a 500 GB WD Passport Elite and a 16 GB Corsair Flash Drive.