The following a very interesting post from Trelawne Equine's blog (Trelawne Equine are the UK distributor of the Easycare range of hoof boots)
A barefoot horses hoof differs in many ways from a shod horses foot, but one noticeable and very important feature is the way the hoof loads when it hits the ground. A naturally bare hoof will support the weight and shock accross the whole foot, unlike a shod horse that loads peripherally (on the wall of the hoof alone). The hoof wall was never designed to take the whole load, and unless the shod horse is working in a fairly deep surface it will miss out on this important additional support. Other than on a totally flat surface such as a road (which it is advisable to only walk or gentle trotting uphill due to the shock involved, although this is obviously substantially less bare hooved than shod!).
When using a hoof boot, you are creating a flat surface for the hoof to walk on, no matter what the terrain. In order to counteract the potential for peripheral loading, it is advisable to always use a pad, and replace them when they become too flat to support the sole. A 6mm medium or soft comfort pad is ideal for horses that have no preexisting problems, and can be used in any of the boots including the Easyboot Glove.
If the horse is sore or has limb problems including minor things such as windgalls or sore shins as 12mm pad would be beneficial. All riding hoof boots other than the glove will take a 12mm pad without the need to alter the sizing.