I have used these packs exclusively for 3 years and hundreds of miles with my two border collies. Kye is a 34lb female and Cody is a 40lb male.When first buying these packs I bought the small for Kye and the medium for Cody, according to the measurements and weight guidelines on the website. Despite this the medium pack really was too large for Cody and I had it cinched down as tight as possible to make it fit. The panniers themselves were massive on him. Recently I replaced his medium pack with a small and it fits much better and has smaller panniers.

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