If you are looking for the most compact version of Ubuntu to save bandwidth or storage, use these official methods:

If you only need the Ubuntu user-space (no kernel), using debootstrap can create a base image of 80-100MB . Beware of "Highly Compressed" Scams

~260 MB (the smallest official stable release).

However, you can achieve extremely small footprints through specific minimal versions and advanced compression techniques. The Reality of Ubuntu File Sizes

At approximately 40MB , this is the smallest "bootable" file available. It provides a text-based installer that fetches only the packages you need from online archives, allowing for a tailored, lightweight system.

~40 MB (this is only an installer that downloads the rest of the OS during setup). How to Get the Smallest Possible Ubuntu