Since the only constant is the watts, transformers are rated in kVA. Let's assume a power factor of 1.0 to make the math easier.
50kVA transformer will handle 50,000 watts.
That's 50,000 watts (416.67Amps) on the 120V side and 50,000 watts (4166.67Amps on the 12V side. The wire is sized to handle the Amps. When you look at any transformer, the Amps on the high voltage side will be less than the Amps on the low voltage side. Thus, the difference in wire sizes.
