re: fractions
Technically, 2/3 is 33% bigger than 1/2, even though they're only .17 from each other on a number line.
Thinking in terms of relative percent, as opposed to absolute difference, tends to give a better idea of the relationship. This is why the difference between a 10 year old and an 8 year old (25% older) is so different than the difference between a 52 year old and a 50 year old (4% older), even though both pairs are separated by 2 years.
[Metacomment: Can you tell I used to teach math?]