The advantage of a small enclosure is that it doesn't take much to passively cool it. Rather than having heatsinks inside the enclosure, the enclosure itself can be the heatsink. Many manufacturers have caught on to this, including Silverstone.
Intel's NUC (Next Unit of Computing) is a small form factor computer. It has everything that you would expect from a modern computer, a handful of USB ports, HDMI output and more. It's just a desktop in a much smaller enclosure. Regardless, it still produces heat, and Intel's reference design for the NUC enclusure therefore includes a fan.
Silverstone has decided to exploit the small form factor and do away with the fans entirely. Rather than having heatsinks inside an enclosure, which would still require fans for some internal airflow, Intel's NUC form factor makes it possible to make the entire enclosure into a heatsink. This does away with the need for fans entirely as there will always be some air moving throughout the room, and even if there isn't, the heat won't get trapped in a small space.
Whilst Silverstone's NUC enclosure will be a tad larger than Intel's reference design, it will still be much smaller than the average desktop. Intel's reference design measures at just over 4 inches width and depth, so anything slightly larger than that is still very small by today's standards.
Silverstone's NUC enclosure is still in the prototype phase though, so no information is available on availability or pricing yet.