Thick insulated copper wires are used in the primary coil of a step-up transformer and in the secondary of a step-down transformer. What is the necessity of this ?
The power in the primary and secondary of a transformer is equal. Current is more where voltage is less because $$P = VI.$$ Thick wires are needed to flow more quantity of current. In the primary of step up and in the secondary of step down, quantity of current is more.