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'Small', 'simple' and 'secure' are values that have been mentioned for Alpine Linux. For size considerations among others, Docker is moving its base image from Ubuntu to Alpine Linux.
Alpine Linux is a lightweight non-commercial Linux distribution based on musl libc and busybox. Although not really intended for a desktop audience, with some tweaks it can be made a fully functional Desktop OS.
Read Alpine Linux on the desktop: awesome feat or fool's errand?.
A container can be as small as 8 MB and a minimal installation to disk requires around 130 MB of storage. It uses its own package manager called APK.
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Pros
Cons
Con Desktop Environment not included by default
The flip side to having the 'small' and 'simple' arrangement here is that the live images do not ship with a desktop environment. Some additional steps are required to install a DE (e.g. Xfce). This may make the installation too difficult for Linux beginners without an understanding of xinit, xorg, etc.