Why do they make half orthotics and full orthotics and what is the difference it makes to the foot or arch I pronate?

Foot anatomy. The orthotic shell has to stop behind the metatarsal heads or you wouldn't be able to push off during gait. If the device was well past the metatarsal heads it would break when you walk. The topcover, however, can be on the shell only or full lenght. Many patients like them full length because they stay in place better when in the shoes.
No diff in support. The most important part of the orthotic is from the heel to behind the metatarsal heads where the rigid support ends. This is what controls the motion in the foot. It makes minimal diiference if it is full or 3/4 length.The padding beyond this is for comfort and cushining and is a option for those who like the feel of a full lenght orthotic.
Orthotics. While not an expert in this area, I have personally tried both myself, the half and the full ones. I do just fine with the half orthotic, but i imagine certain conditions are best handled with the full ones. Also, i think i chose the half, in part, because they were less expensive. As you probably know, custom, quality orthotics can be costly. I would check with a podiatrist. Good luck.