Sorry this is a somewhat handwavy explanation that glosses over many details, but the size of components in electronic circuits (i.e. transistors and connections) fundamentally depends on the de Broglie wavelength of the electrons (some tens/hundreds of pm) why the size of a photonic circuit depends on the wavelength of light (order of 1 mu m). So one can make much more compact electronic circuits than photonic. Note that is somewhat different to transmission lines which are larger for electronics (depending on the RF carrier frequency) than optics.