I think what you're on about is BOCU, Binary-Ordered Compression for Unicode or one of its predecessors/successors. In particular,
The basic structure of BOCU is simple. In compressing a sequence of code points, you subtract the last code point from the current code point, producing a signed delta value that can range from -10FFFF to 10FFFF. The delta is then encoded in a series of bytes. Small differences are encoded in a small number of bytes; larger differences are encoded in a successively larger number of bytes.