I think what your trying to say is that your using TinyPNG to lossy compress your PNG files before importing them to C3. But that it's having no effect on the export size of the project.
This is expected, TinyPNG works by converting the images to 8 bit indexed PNG files ( 256 colours ). However, when C3 exports images it combines them into spritesheets in the standard PNG format ( 32 bit - 4.2 billion colours ).
Choosing the option "re-compress images" in the export should losslessly reduce the size of your images, this is a good starting point. Alternatively you can run TinyPNG on the Spritesheets after export if you want, but given that you will have many images on the same sheet reducing it to 256 colours may cause issues with image quality.
Reducing the physical dimensions of your source images is generally the best approach to reducing memory usage.