B is 13-17 times lower/smaller than A
The problem is then, how would one best rewrite this since "x times" will, at least mathematically when x > 1, always be larger and not smaller?
I disagree with the premise of the question. The example is OK, although a little awkward. In "x times smaller," the word "smaller" inverts the ratio. There are of course other ways of expressing this, but that's a matter of style. Using "by a factor of" doesn't necessarily fix things:
(1) A millimeter is ten times smaller than a centimeter.
(2) A millimeter is smaller than a centimeter by a factor of 10.
(3) A millimeter is smaller than a centimeter by a factor of 0.1.
(4) My car is lighter than your car by a factor of x.
Of 1-3, I think 1 is the best style and is fine mathematically. 2 and 3 show that there's a potential ambiguity. In 4, the ambiguity is a real problem, because we can't tell if x is being defined as a number that's less than 1, or greater than 1.
Edwin Ashworth wrote:
[...] linear scale factors are used to avoid [...] the confusion where in everyday language 'ten times bigger' is used to mean 'x10' whereas 'one time(/s) bigger' (paraphrasing 100% bigger) means 'x2'
I think this is an innumeracy issue, not a language issue. Some people just don't understand how to convert back and forth between ratios and fractional changes, or don't realize that they're different things. For example, if they're told that B is 7% greater than A, and are then asked to find the ratio B/A, they may say 0.07. We're talking about scientific writing, where this kind of innumeracy isn't an issue.