Reason is in density of electrical field in your case in oil, but it can be any fluid or material. Bigger electrode means less density of electrical field on a surface of electrode. That is a reason why each shape in inside transformer or on outside equipment (like bushing) always have some smooth surface and shape. There isn’t any sharp edge or peak. That is why many research works is done for electrical and magnetic field for electrical equipment in general. This more interesting when the rated voltage of equipment is higher. The complexity of problem started with 400 kV.
A metal sphere (radius R) has the capacity C = 4 * pi * eps * R. Applying the voltage U, it carries the charge Q = C * U. The electric field strength at the surface is E = Q / (4 * pi * eps * r²). If one combines these formulas, one finds: E = U / R. For a given voltage U, the field strength decreases with increasing radius. This reduces the risk of corona discharge and electrical breakdown.
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