Same issue with Large Cartels and size 12 IONs. The gap is pretty much the same. I havent noticed any movement of the boot inside the binding, as long as the straps are tight enough. I can force some slight movement if i try really hard, but only intentionally. I dont think the gap can be avoided in large bindings, unless you were a boot of a size like 14.
However, if it is easy to try medium cartels and swap them if they fit better, do it, otherwise i wouldnt bother.