Why etc2 & pvr is so large?
Indeed file sizes for compressed textures are big! But they do compress nicely using gzip compression, so make sure to have that enabled in your server.
The file sizes aren’t representative of the video memory they occupy when uploaded to the GPU.
What MIME type needs for ktx files compressing?
OpenGL Textures image/ktx .ktx ?
I think so yes, this is from the GLB parser but the mimetype should be universal: