With mac I am able to bring heap size to 9600M that was enough to use webp compression for images. I or Defold is doing something wrong with it, because I ended up with 1,3GB apk with that option. When I have little more time, I will test more with it.
With Windows machine after growing the heap size to 2400M, Defold did not open anymore. That might be device dependant issue. I will try that when I am back home with my own device.