Seagate's Backup Plus Fast manages to pack 4TB of storage and good transfer speeds into a compact case that you can take almost anywhere. At 0.8-inches thick it may not be as small as leading competitors, but CNET doesn't have an issue with the added bulk, as the Backup Plus Fast is "still very compact."
That extra bit of real estate does have its benefit, though -- by putting two 2TB hard drives in a RAID 0 configuration, the Backup Plus Fast delivers "zippy write speeds and average read speeds" according to Computer Shopper, and PC Mag says it's "one of the faster large-capacity external hard drives" they tested.
Unfortunately, StorageReview says the drives aren't "user accessible," putting you at the mercy of a support ticket should one of the drives fail. Still, fast speeds and small size make the Backup Plus Fast a good option for carrying around large bundles of data if you move around a lot -- just don't rely on it as your primary form of storage.