Strange indeed that Middlebury isn’t on this list. In the combined university and LAC ranking, Middlebury is #52, directly behind #51 Wesleyan. So if included in this list, it would come in at #12.
Also, in Forbes’ profile of Middlebury, they state that Midd has a student population of 4,813, including 4,137 undergrads. What might be at play here is a change in how enrollment is calculated, which party accounted for Midd’s decline in USNews last year (as it likely will again this year).
They may be including all the students who attend the summer language schools, Bread Loaf School of English, and MIIS in California.