>

Wait until you see costco size pizza that is cheaper than a small dominoes
This is a friendly reminder to read our rules.
Remember, /r/Showerthoughts is for showerthoughts, not "thoughts had in the shower!"
(For an explanation of what a "showerthought" is, please read this page.)
Rule-breaking posts may result in bans. >

If I eat a quarter of the 12 inch instead of the 10 inch, am I eating 7.5% more? Or 30% more? >

30%
If a 12-inch pizza is 130% of a 10-inch pizza, then any fraction of the 12-inch pizza is 130% of the same fraction of the 10-inch pizza. >

That's one way to look at it...or you can think of it as a 10 inch pizza being 30% smaller than a 12 inch pizza! >

Actually, if a 12-inch pizza is 130% of a 10-inch pizza, a 10-inch pizza is about 77% of a 12-inch, so 23% smaller. >

This is astute >

I'm feeling dumb can someone tell me why it's not 20% larger... >

The area of the pizza is a 2-dimensional property, length and width. A bigger diameter increases both dimensions and so the increase applies two-fold. Because by the definition of a circle, the length and width are always the same.
Also, 30% bigger is technically incorrect. A pizza with 1.2x the diameter, multiplied by itself:
1.2 x 1.2 = 1.44
The 2nd pizza is 44% bigger than the first. The first pizza is 30% smaller than the second.
It's weird that these numbers are different, but it's because the actual amount of pizza that changes is bigger relative to the smaller pizza. Like how doubling something is a 100% increase, but halving is a 50% decrease.
144 / 100 =1.44
100 / 144 = \~ 0.7
Bonus: Since the thickness of the crust doesn't scale to the size of the pizza, the amount of crust will increase at a different rate to the rest and it will have an even bigger increase of cheese and toppings. 
Assume a 1" thick crust and that the numbers given are actually the radius, because that's what you use to calculate area.
10^(2) = 100
9^(2 =) 81 interior
100 - 81 = 19 crust
12^(2) = 144
11^(2) = 121 interior
144 - 121 = 23 crust
23 / 19  = \~21% increase of outer crust
121 / 81 = \~49% increase of interior
By the way, all these numbers should be multiplied by π to get an actual measurement in inches. But since we're only comparing the two sizes and they all just get multiplied by the same thing, we can leave it out. >

Um. You win. >

Yeah? But how many more pieces do you get? That's really what matters!
/s >

Unironically the size difference in slices is crazy. My sister works at domino’s and a small slice is probably 4x smaller than a large and it’s only like 2$ cheaper for an entire small