Indeed, if you search for "ceramic antennas" you'll see that they are already being used and smaller than equivalent PCB antennas. They rely on a dielectric mat

Indeed, if you search for "ceramic antennas" you'll see that they are already being used and smaller than equivalent PCB antennas. They rely on a dielectric material with high e_r, which implies that speed of light is slower there. Lots of portable devices use them nowadays!