Howdy.
Average is like defining the population of military recruits,one size fits all.
Small market hogs seem to be going 240-280 lbs. and they tend to produce a butt between 5-6 lbs.
This is a pretty nice size, for cooking several for competition,because they are a little younger and the size finishes a little sooner- to be sure you have turn in on time.
If the producer can afford to hold them a little longer,they finish at about 320 lbs. and are more profitable.
These seem to produce butts between 7-8 lbs.
These are my preference for home cooking and are usually what is sent to the stores in a 2 pack in cryovac.
Older bearing sows[sausage sows],stags, and old boars may run 700 lbs. and can produce butts up to about 12-14 lbs.
Think older ,more dry,tough,etc.
Meat retailers tend to be able to sell four lb. trimmed roasts to the family cook,that will go home and braise them.
Ask any meat cutter, that is selling country style ribs on promo,if he has any 2 packs of butts in the cooler.
He should be willing to sell them at promotional rib prices,since he doesn't have the labor of trimming , slicing,and repackaging.
Hope this helps a little.