After doing a little reading, I learned that US News dropped class size (and 5 other variables) because they were self-reported by schools, and many large schools had stopped reporting this metric (because it was hurting their ranking). US News tried to compensate for this by increasing the weight of the student:faculty ratio, but they only increased that a little, and has been pointed out, that doesn't measure the same thing as class size.
So, US News didn't drop this metric because they found it unimportant, they dropped it because they couldn't get large schools to report it. Their diplomatic phrasing, "Five variables were droppeed: class size (8%), the proportion of a school's faculty with terminal degrees (3%), alumni giving rate (3%), the proportion of graduates borrowing (2%) and high school class standing (2%). Although each of these statistics adhered to industry standard definitions from the Common Data Set and the Council for Advancement and Support of Education (CASE), they were not collected or computed by the U.S. Department of Education and therefore not as universally reported by schools."