Why small PCs should be fanless
A PC usually relies on heat sinks and active cooling by forced airflow (ie fans) to remove heat from hot components. As the case becomes smaller so does the fan. As a fan becomes smaller it needs to rotate faster to achieve the same airflow. Faster rotation results in higher noise level and higher wearing rate. Small cases has small air inlets that clog quickly and decrease airflow efficiency which require the fan to rotate faster increasing noise, wearing and clogging rate further. Therefore small PCs that rely on fans for cooling are prone to overheating.
Passive Cooling and fit-PC2
Passive cooling by conduction is silent and has no moving parts that can fail. It is also less efficient than active cooling. The accomplishment in fit-PC2 is bringing the system power consumption to such a low level as to make passive cooling in a small case possible.