Each size is optimized to reduce bulk, allow easy access to contents and eliminate interference. New sizing requires moving down a size from previous Pro Pack. Previous Pro M 50 replacement would be new Pro Pack S, etc….. Non-QC packs stay firmly attached to the saddle rails with V-LOC mounting system. QC versions use the Interchange Quick Cleat system for easy attachment to most saddle rails. Elastic band on bottom for mini pump on XS, S and M. Internal pockets on all except XS. Main opening gusset to keep contents in place.