We investigate the operation of pyramidal magneto-optical traps (MOTs) microfabricated in silicon. Measurements of the loading and loss rates give insights into the role of the nearby surface in the MOT dynamics. Studies of the fluorescence versus laser frequency and intensity allow us to develop a simple theory of operation. The number of ⁸⁵Rb atoms trapped in the pyramid is approximately L⁶, where Llsim6 is the size of the pyramid opening in mm. This follows quite naturally from the relation between capture velocity and size and differs from the L^3.6 often used for describing larger MOTs. Our results represent substantial progress towards fully integrated atomic physics experiments and devices.