WOFF is simply gzip plus a header. This works well for OTF fonts where the format is already heavily optimised for low size. However TTF files don't compress as

WOFF is simply gzip plus a header. This works well for OTF fonts where the format is already heavily optimised for low size. However TTF files don't compress as well, particularly the 'glyf' table whic contains the outlines and typically accounts for a large percentage of the file size. WOFF 2 adds a preprocessing stage which rewrites the glyf table into a more compact format prior to gzipping.