The Burton L-Bindings are surprisingly big. I use the center mounting holes with my size 14 reduced foot-print boots. The non-reflex versions had a lot more front-back adjustment range - that's working against you as well.
The binding is definitely big for your boots, if you can easily exchange it against a medium, you should probably do that.
It's normal that the highback doesn't fit the heel-loop perfectly, especially if you rotate it. But it should be close to touching at the forward lean adjustment screw - hard to tell from the picture (push it back as much as possible into the heel-loop).