The key to slug shooting is getting very solid shoulder-pocket placement with the stock (a Limbsaver or Kick-Eez pad doesn't hurt). Along with that, a PG forms a very solid triangle that allows some downward-pull in recoil. Generally, most folks can get a good pocket, with practice, and maybe a couple of pointers. My suspicion is that if you're a large-framed guy, you may not have been forced to get that placement down until stepping up to a load that overcomes what you can easily absorb with your arms, i.e., you may be doing a bicep/forearm flex more than pulling into your shoulder.