To get the highest possible capacitance we need to get the plates as close together as possible.
Electrolytic capacitors use a special technique to achieve a plate spacing which is very small.
They're made in two stages. First, fill the gap between the plates with an electrolyte . Then 'form' the capacitor by applying a d.c. voltage. The electrolyte conducts, carrying charge between the plates. This stimulates a chemical reaction at one plate surface which forms a layer of insulating material.