This is an easy fix! Lots of people who have muscular calf muscles have had this issue. No need to panic. There are two things that must look correct when the shins are worn. The front center line must be centerline with your leg and the back raised vertical strip needs to be centerline with your leg. The length of your shins looks good you just need to move the whole thing down over the boot.
Your first decision will be are you willing to remove the padding from the tongue of the boot and/or the padding that is in the top of the ankle pads. I originally sized my shins based on the Talib boot which has no padding at all, so when I switched to the Jared they did not close. I cut openings on the inside surfaces of the tongue and ankle pads and removed the padding. They will still meet L2 requirements as the structure and form still remain "Jared." The only thing removing or not removing will effect is how much of an extension strip you will need to make so that the top and bottom of the shins close.
No need to mess with your front because its a solid piece, you are at an advantage because you chose to clamshell close your shins at the back.
Here's how you fix it: You remove the rear vertical raised center strip, add in extensions to the left and right edges so that the sides close and the outside edge overlaps the inside edge the same width as the vertical center strip. Affix the center strip aligned with the center line of your leg and Wahla! your shins close and meet CRL requirements. You will have to build up any details that continue from the original piece to the center vertical strip, but bondo, filler putty, or an ABS strip can take care of the extension of the details.
Gerald @DoggyDoc had to make extensions for his forearms and shins and he has a really good section on it in his build thread.
Here is a simplified explanation I did for someone a while back which may help in visually seeing what I suggested.
Hope this helps. Like I said don't panic. Also, Jim only makes 2 sizes, regular and tall, neither one increase the diameter of the shins, only the length is different.