2.5″ laptop drives are designed to run with a lot less power than 3.5″ desktop drives because they’re usually used on battery power. They’re slower, but a lot less power-hungry. If the external enclosure you’re buying contains a 2.5″ drive then the amount of power that can be delivered over a USB connection is sufficient to run the drive. But if the external enclosure has a 3.5″ drive inside it then USB can’t provide enough power to run it and you need to plug it in to a power outlet using one of those “power bricks”.
It’s not just power difference between 3.5″ and 2.5″. It is the voltage requirements. 2.5″ drives require just 5 Volts (reason they can work off of a USB port AS long as the current does not exceed 0.5 Amps) However a 3.5″ HDD requires both +12 V and +5 V. Normally +12 V is supplied from the “Brick” to the HDD enclosure then inside the Enclosure +12 V goes to the drive and to a step down circuit that derives the needed +5 V. If you are looking for portability (NO AC brick), you can get a recharable +12 V battery you could use that to power the HDD enclosure – BUT DO NOT recommend doing this as if the batteries dies while writing to the drive you could loose the drive.