"Small gods" is a term which has a number of meanings, at least in the Morporkian-speaking Discworld. As with the Shades of Ankh-Morpork, it's relative. There are uncounted millions of would-be gods, placeholders for gods, seeds of gods who lack the one essential of godhood: belief. In some places (and they're usually desert places - it seems to be a Multiversal convention) they swarm in boiling clouds, pleading and whining for the first believer, just one!
Gods who are comparatively strong and important may still be small compared to the High and Mighty of Dunmanifestin. This group includes all the known small gods, from Herne the Hunted to Anoia: a recent success story. The better known of these share space at the Temple of Small Gods, often called "Small Gods", on the Street of Small Gods.
As they grow in strength proportionately to the available belief, so do they fade if they lose their believers to other gods, natural disasters, religious wars, etc. Many of the once-important gods are so "small", they are only whispers in the wind.