When you upload a piece of content to our servers, we run a variety of processes on it before building the final export packages. We build one package per platform (e.g. one for WebGL, one for standalone, one for each mobile OS, one for each console.)
Depending on the platform your textures are likely to be re-compressed into a native format for that platform, e.g. DXT1/5 on Standalone, JPEG for WebGL, etc.