From the picture of your skates, I can't imagine a small chassis has a shorter wheel base at all as there is not much room between your wheels. I would guess, the difference between a small and medium would be where the mounting points are. Does that make sense? FYI, I know that the small Vanguard chassis on the Soldier series skate uses the 80/76mm combination. Good luck! By the way, the skates look great...I think the medium chassis was the right call. Thanks, my thoughts mirror yours about the mounting points being different as I couldn't think of a way in which they could make any of the other dimensions smaller.