Google is now compressing its online fonts using Zopfli, an open-source compression algorithm. That produces a size reduction of between 6 per cent and 15 per cent — notable but not enormous — so why did it make the switch?
As Google developer Ilya Grigorik explains in a Google+ post, even a small size reduction is beneficial in terms of download speeds. " For example, Open Sans is one of the most popular fonts and is now more than 10% smaller — this translates to faster load times and faster rendering," he wrote.
Zopfli was released by Google early last year. It's much more processor-intensive when compressing (since it effectively calculates multiple possible different compressed files and chooses the smallest one), but can be decompressed by existing software. That means it works well for content that doesn't change, like fonts, but wouldn't be as beneficial for extremely large files such as movies.