how to make high quality textures with low size of downlaod

Hi
now i am about to finish my game for google play store
but the textures quality is so low
when i make it high or true color
the size become so huge

and i saw a lot of games on the market has high quality
and the same time has low size for download
so i want to do the same
so what is the concept of this ?

Texture compression for mobile generally lowers the amount of colourspace (bits per colour goes from 8 to 4, for example). A good image editing tool will be able to apply dithering to the image. Floyd-Steinberg dithering can give you perfect-looking gradients when pre-applying colour reduction to your images. Texture Packer can also do this.