Hi all,
Rob Taglang just wrote a Cesium blog post about compressing glTF and 3D Tiles with the WEB3D_quantized_attributes extension:
http://cesiumjs.org/2016/08/08/Cesium-web3d-quantized-attributes/
This is basically free compression, just check the box in the online model converter and use Cesium 1.23 or later. It is most effective for geometry-heavy models with no textures, where it can produce ~15% savings (comparing gzipped to gzipped).
Patrick