The electronic logic elements that they are made of got smaller and smaller. First they were electric relays (about the size of a ping pong ball, then vacuum tubes, then transistors. The transistors consist of materials that conduct electricity under certain conditions. The transistors became smaller and smaller. A single silicon chip can now contain many millions of transistors, each of which has the function of one electric relay. The limit is the (three dimensional) geometry of the transistors on the chip and the (photographic/deposition) process used to make them.