The former is 0.1 the latter is 0.002

My cursory research on the topic suggests that while the ratio itself is a fraction, the "size" of the ratio refers to the inverse: the magnitude of difference between numerator and denominator, with 1:1 being the lowest ratio.

This seems to be more of a linguistic thing than a math thing, which makes this thread on-topic again. :D

If we are going to talk linguistics, then saying "the cost/benefit ratio improves" is probably more clear than saying it gets larger or smaller then.