It comes down to convention. When talking about proportional differences, we can fix the unitary example to be the smaller, larger, earlier, or later, object or subject.
I think, generally, we fix on the earlier when talking about the change over time of a characteristic. "This stock went up 100%, this stock went down 50%". In both cases it's the earlier measurement that is taken as the unit. That makes this a 45% reduction in time to do the work, and that's actually what they measured.
When talking about comparisons between two things that aren't time dependent it depends on if we talk in multiples or percents I think. A twin bed is half as big as a king bed. A king bed is twice as big as a twin bed. Both are idiomatic. A king bed is 100% bigger than a twin bed. Yes, you could talk like this. A twin bed is 100% smaller than a king bed. Right away you say wait, a twin bed isn't 0 size! Because we don't talk in terms of the smaller thing when talking about decreasing percents, only increasing. A twin bed is 50% smaller than a king bed (iffy). A twin bed is 50% as big as a king bed. There, that's idiomatic again.
I think, generally, we fix on the earlier when talking about the change over time of a characteristic. "This stock went up 100%, this stock went down 50%". In both cases it's the earlier measurement that is taken as the unit. That makes this a 45% reduction in time to do the work, and that's actually what they measured.
When talking about comparisons between two things that aren't time dependent it depends on if we talk in multiples or percents I think. A twin bed is half as big as a king bed. A king bed is twice as big as a twin bed. Both are idiomatic. A king bed is 100% bigger than a twin bed. Yes, you could talk like this. A twin bed is 100% smaller than a king bed. Right away you say wait, a twin bed isn't 0 size! Because we don't talk in terms of the smaller thing when talking about decreasing percents, only increasing. A twin bed is 50% smaller than a king bed (iffy). A twin bed is 50% as big as a king bed. There, that's idiomatic again.