Yeah but how often does armor and shielding actually get used? Either way, a big gun may seem like the better option, but by firing two smaller weapons you have a 1/3 chance for at least one of them to make a crit, and punching right through the armor.
If you ask me, 10% is a pretty significant difference. We're only comparing 1" weapons to 2" weapons. I'm too lazy to see if the math gets exacerbated when you go up to, say, a 3" gun and a 6" gun.