1. KolibriOS: Smallest GUI OS. Kolibri is the smallest GUI operating system. It was forked off from MenuetOS. Written completely in assembly language, it is available in two versions: 1.44MB with essential features and 3MB with additional features.lax is a modern, live, pocket operating system based on Slackware Linux. Despite its small size of 220MB, it contains essential apps for basic computing, and you can extend its functionality using modules.