When I think of shoulder, I think of both the Boston Butt and the Picnic combined, which is why a whole shoulder weighs about 12 to 15 pounds. A picnic and a boston butt would weigh a lot less.
This is from a FAQ I just did a google search on:
What's the difference between a pork shoulder and a picnic shoulder?
A pork shoulder is the shoulder and front leg of the pig. The picnic shoulder is a portion of that cut. The picnic shoulder (or picnic shank)and Boston Butt (often called the blade roast or button roast) together make the shoulder (often called a "full" or "barbecue" shoulder). Different areas of the country have different names for these roasts, and there is little standardization in that area. If you are in doubt as to the exact cut of meat you are getting, ask you butcher and tell him/her what exactly you're looking for.
A full shoulder weighs in around 12 - 16 pounds, but can go as high as 25. The picnic portion of the shoulder is the portion starting just above the knuckle joint and extending upwards to where the shank bone joins the body of the pig. The Boston Butt portion is the actual shoulder of the pig, the meat mass surrounding the shoulder joint. A full shoulder will contain the arm bone, shank bone, and a portion of the blade bone. The picnic shoulder will contain the arm and shank bone; the Boston Butt will contain only the blade bone.
“If you don’t know who I am, then maybe your best course would be to tread lightly.”
For links to all my BBQ Brethren recipe posts, click http://markandteri.com/recipes.htm