As the winding wire gets thinner, the amount of heat generated by the increased resistance increases, so the windings overheat. So if you use thinner wires then the current rating falls.
Frank
You can use thinner wire but you need more parallel cables. Thinner wires = more parallel cables, thicker wires = less parallel cables. You need to use skin depth formula because of skin effect. You must avoid using thicker wire according to this formula.