It's an awful amount of characters once you multiply it. For instance, the first example (https://tailwindcss.com/docs/what-is-tailwind/) would be twice as big:

    <img class="block h-16 sm:h-24 rounded-full mx-auto mb-4 sm:mb-0 sm:mr-4 sm:ml-0"

    <img class="block height-16 small:height-24 rounded-full margin-horizontal-auto margin-bottom-4 small:margin-bottom-0 small:margin-right-4 small:margin-left-0"

After spending more than, say, two hours with Tailwind you'll probably read the first line much faster than second one (since there's less to read and you don't need to scroll). Humans are very good at pattern recognition, and you'll quickly read "mb" as a symbol in itself and not as "m[argin] b[ottom]".

> I can't believe that mnemonic based languages are easier and more readable than something closer to a natural language.

Well, it's worked out pretty well in mathematics. This is not about file size at all. It's about defining an "alphabet" of common concepts.