We put 16" wheels on our bikes primarily for the obvious reason - they allow a compact folded package. However, smaller wheels bring other advantages: they offer superior acceleration and lose less speed to wind resistance and, when fitted with high pressure tyres (as Bromptons are), the actual rolling resistance is similar to that of large wheels; the unpaced (i.e. unassisted) cycling world speed record was set using a small-wheeled Moulton AM7 (51.29 mph over 200m in 1986).
Smaller wheels are also lighter and stronger than larger, longer-spoked wheels, but many people worry about their capacity to handle potholes and other unforgiving terrains. Despite appearances, our wheels fare pretty well on pitted streets and, because of their greater manoeuvrability, it’s easier to avoid the obstacles in the first place. The only times you have to take more care with small wheels is on a gritty surface or when riding obliquely across a groove or step in the wet, circumstances which require care on any bicycle in any event.