Well I'd rather say it's 4 times the size of 8, but I certainly don't say that 6 is "75% more than 8"

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I also wouldn't say that 6 is "75% less than 8" if your thought is that "more" just indicates a bigger value.

6 is 25% less than 8 - it's 75% of 8.

I agree; and similarly 10 is 25% more than 8 and 125% of 8.