You guys are going to hate me, I'm talking premiums.

Quick question, if "premium" on a 1/2 goldback is double, ex. 1/2 value = 4.21 and 1/2000 of gold melt value= 2.11 why do larger demonization goldbacks have wider spreads? Shouldn't a 100 goldback = $421 melt value have a similar premium of value of $430?