This is because the angle of the Cartel highback is not the same as the angle of the calves-part of the boot. You can of course vary the angle of the highback, but it is set as much upright as possible, already. If I'd vary it, the highback would be more "horizontally" and the space between the heel and the heel cup of the binding would be even larger.
Up to now I thought that this wouldn't be a problem because at the upper part the boot presses against the highback and so I can put force to the backside. So you think that this is a major problem?
Maybe the problem persists with a Cartel in size M because the angle of the highback stays almost the same, doesn't it? I had a Cartel in M before, but I gave it back because I felt like the straps were a bit too short for the boots. Maybe I have to change to some other binding. Could you recommend one? Should be good for all-mountain riding. I looked at the Rome 390 Boss before and maybe I'll get them. I also like the idea of canting pads because I have some knee issues.
You should force the boot all the way back against the heelcup, the upper part of the boot will just flex forward from the highback as needed. It's a little hard to tell from the pics with your boot loose in the bindings, but it does look to me like the bindings might be a little big for the board. It would be easier to judge that without the boots there at all. If you have room to shift the bindings towards the toeside edge a little without significant footbed overhang, then it might be ok.