And ideal antenna is dipole (two sticks) of quarter wave length.
In short, vacuum have coefficients 1 for electric and for magnetic properties (Earth atmosphere is very close to vacuum) and in medium with higher coefficient (all other mediums have coef 1 or more), speed of light is smaller and wave length is also smaller.
Most known for e-m props are glass, water and glycerine (or just commodity oils), I at the moment lazy to look for glass properties (they vary), but for glycerine, electrostatic 80 times more than vacuum, so antenna will be 80 times smaller.
And after Second World War found (invented) new materials, which have e-m coef-s of 1000-10000 and even more, so could make much smaller antennas than for free air.
Also exists lot of variants of spiral antennas, which are less effective than ideal dipoles, but good enough for practical purposes.
In most radical forms, used some sort of spiral coated with material with very high e-m coefficient, so antenna could be very small.