The 2.5-inch form factor is considered small, but there's an even smaller option for solid-state drives and hard disk drives: 1.8-inch, which is used by the latest Team Group product.
Said product is called PD200 ultra-speed and is a portable drive with a USB 3.0 port and a speed 2.3 times faster than normal USB 3.0 external SSDs/HDDs.
By extension, said speed is 11 times better than that of the traditional USB 3.0 flash drives.
For those who want the numbers, the read speed is of 250 MB/s, while the write speed is of 120 MB/s.
For something that is just 8.6 mm thick, and almost as small as a flash drive itself, that's quite the transfer rate.
Team Group has prepared 64GB, 128GB and 256GB PD200 models, all with leather holsters and cable lines. Prices are a mystery though.