The larger the pizza, the smaller the ratio of fixed width perimeter to center. If the area of the perimeter were to maintain a constant ratio to the total area, it's width would have to increase in proportion. That applies to any 2D shape, not just a circle or square.
In fact, the calculation in op is wrong because a 16" pizza isn't merely four times as large as an 8" pizza. The 1" crust makes the 16" really a 14" while the 8" is really a 6". 14^2/6^2 = 5.44 times as large.
b: If you were to say anything like that in the USA, you'd be reported to the TSA for un-American activities as soon as anyone heard you. And the NSA would hear you, even if you said it inside your own head. So bring a 100ml or smaller bottle of lube with you for the probe next time you fly.
b) When it comes to food, money should not be the only factor in the equation. There is the health aspect as well, and if you are not that hungry, you better off eat what you need, rather than eat more just because it is cheaper.
This applies on all junk/industrial/processed food - the less you consume, the better!
Your health is important at least as your wealth.
Another example, if you are thirsty and a can of Coke is enough, get the can, despite the fact a larger bottle has x5 drink in it and cost few cents more, simply because a bottler will make you drink more Coke, while you should drink less.
[1] http://www.tripadvisor.com/Restaurant_Review-g293984-d807042...