If I understand correctly gzip, brotli and similar are best used to compress text.

Compression algos should be used on uncompressed data. Using them on already compressed data (most video, images, music formats) is generally useless.

Why 28Kb though?

It’s from an era where everyone just used open standards instead of trying to create their own thing for money.

SMS is literally from a time when every mobile phone manufacturer had their on charger plug. And some tried pushing proprietary headphone jacks.

Vendors LOVE vendor lock-in.