I put the smalls on Izzy just for comparison, and I could barely hook the velcro to the other side. He wears a medium in the front which fits perfectly as well.
The small and medium sized boots close with two velcro straps, but the large-sized boots use three velcro straps. It makes it easier to tell which is which.
Obviously I couldn't resist.
For the price, I can afford to have several pairs for each boy. Would three pairs per horse be just silly? The navy is calling my name ...