Everyone has the die state issue correct. But the question was what caused it. when a die wears, there are often certain areas that are affected first. Here is a set of images of the different die states and how the outside edges of the devices are affected:

Note the slight differences to the outside edge devices near the rim? Little noticed in the earlier die states, but as the die ages, it is more noticed. The devices get taller a bit, wider a bit because of die wear. Also not the fields of your coin? Note the lines between the devices that radiate from the center outward? That is cause by die wear. With the pressure applied during the strikes again the planchets, this will break down the die. The coins show what look like wrinkles on the fields. That will show in the LDS-VLDS die states. So it is because of the high number of coins struck with the die, that makes these devices enlarged. The die that struck your coin is very close to retirement.