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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.
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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!